[NativeAOT] fix default SynchronizationContext (#9883)

Context: #9846
Context: b6c22f3

Running `Mono.Android-Tests` under NativeAOT has the test failure:

	I NUnit   : SynchronizationContext_Is_ThreadingSynchronizationContextCurrent
	E NUnit   : 	[FAIL]
	E NUnit   :  :   Expected: True
	E NUnit   :   But was:  False
	E NUnit   :    at libMono.Android.NET-Tests!<BaseAddress>+0x1482f63
	E NUnit   :    at System.Reflection.DynamicInvokeInfo.Invoke(Object, IntPtr, Object[], BinderBundle, Boolean) + 0xf3

`SynchronizationContext_Is_ThreadingSynchronizationContextCurrent()`
failed because we never called `JNIEnvInit.SetSynchronizationContext()`.

Call `JNIEnvInit.SetSynchronizationContext()`, and…

	F libc    : Fatal signal 6 (SIGABRT), code -1 (SI_QUEUE) in tid 17119 (Thread-11), pid 17058 (droid.NET_Tests)
	I crash_dump64: obtaining output fd from tombstoned, type: kDebuggerdTombstoneProto
	I tombstoned: received crash request for pid 17119
	I crash_dump64: performing dump of process 17058 (target tid = 17119)
	F DEBUG   : *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
	F DEBUG   : Build fingerprint: 'google/panther/panther:14/AP2A.240905.003/12231197:user/release-keys'
	F DEBUG   : Revision: 'MP1.0'
	F DEBUG   : ABI: 'arm64'
	F DEBUG   : Timestamp: 2025-03-07 15:48:39.677241696-0600
	F DEBUG   : Process uptime: 2s
	F DEBUG   : Cmdline: Mono.Android.NET_Tests
	F DEBUG   : pid: 17058, tid: 17119, name: Thread-11  >>> Mono.Android.NET_Tests <<<
	F DEBUG   : uid: 10582
	F DEBUG   : tagged_addr_ctrl: 0000000000000001 (PR_TAGGED_ADDR_ENABLE)
	F DEBUG   : signal 6 (SIGABRT), code -1 (SI_QUEUE), fault addr --------
	F DEBUG   :     x0  0000000000000000  x1  00000000000042df  x2  0000000000000006  x3  0000007c6b4838f0
	F DEBUG   :     x4  0000000000000000  x5  0000000000000000  x6  0000000000000000  x7  0000007f36a58004
	F DEBUG   :     x8  00000000000000f0  x9  0000007f18a6d350  x10 0000000000000001  x11 0000007f18abe170
	F DEBUG   :     x12 0000000000000004  x13 51c34e921a529271  x14 0000000000000001  x15 0000000000000001
	F DEBUG   :     x16 0000007f18b24fd0  x17 0000007f18b10560  x18 0000007c694dc000  x19 00000000000042a2
	F DEBUG   :     x20 00000000000042df  x21 00000000ffffffff  x22 0000001a580093f8  x23 0000007c6b483e18
	F DEBUG   :     x24 0000000000000010  x25 0000000000000014  x26 0000001a5bc27ab8  x27 0000001a5bc27b40
	F DEBUG   :     x28 0000001a56005b38  x29 0000007c6b483970
	F DEBUG   :     lr  0000007f18aa78b8  sp  0000007c6b4838d0  pc  0000007f18aa78e4  pst 0000000000001000
	F DEBUG   : 2 total frames
	F DEBUG   : backtrace:
	F DEBUG   :       #00 pc 000000000005d8e4  /apex/com.android.runtime/lib64/bionic/libc.so (abort+164) (BuildId: 1d36f8ae6e0af6158793abea7d4f4f2b)
	F DEBUG   :       #1 pc 0000000000034828  /data/app/~~rqTsxkU5NjJYcDmnf-0haw==/Mono.Android.NET_Tests-fIi1xEY1rqvY2p4EDO81Ww==/split_config.arm64_v8a.apk (offset 0xa6c000)

After lots of digging around, we discovered that the problem was that
the `Android.App.Application` static constructor was running
*too early*, which caused an `Android.Runtime.XAPeerMembers` instance
to be constructed *before the runtime had finished initialization*.

Which leads us to a digression: when is static constructor run?
The answer to this depends upon the [`beforefieldinit` flag][0]:

  * When `beforefieldinit` is set, the runtime has lots of
    flexibility over when the static constructor is executed.
    The static constructor could be executed far earlier than one may
    normally expect.

  * When `beforefieldinit` *is not* set, the static constructor
    is executed "immediately" before the type is used.

In C#, if a type does not contain a static constructor, then the
type has `beforefieldinit` set:

	// has beforefieldinit
	partial class Application {
	  static readonly JniPeerMembers _members = new XAPeerMembers ("android/app/Application", typeof (Application));
	}

If a type has a static constructor, then it does not have
`beforefieldinit` set:

	// does not have beforefieldinit
	partial class Application {
	  static readonly JniPeerMembers _members = new XAPeerMembers ("android/app/Application", typeof (Application));

	  static Application()
	  {
	  }
	}

We hit this same scenario in b6c22f3 when using Mono's LLVM toolchain,
and fixed that by using `[MethodImpl(MethodImplOptions.NoInlining)]` on
`SetSynchronizationContext()`, in the hope that doing so would delay
execution of the `Application` static constructor until the invocation
of `SetSynchronizationContext()`.  This worked for b6c22f3, but not
with NativeAOT.

To fix this:

  * Add an empty static constructor to `Android.App.Application` to
    remove the `beforefieldinit` flag from that type.

  * Call `JNIEnvInit.SetSynchronizationContext()` at startup for
    NativeAOT.

With this change in place, no crashes occur at startup and the test
passes.

[0]: https://csharpindepth.com/Articles/BeforeFieldInit

Author: jonathanpeppers

src/Mono.Android/Android.App/Application.cs

@@ -8,6 +8,10 @@ namespace Android.App {
 
 	partial class Application {
 
+		static Application () {
+			// NOTE: do not remove, removes the beforefieldinit flag for JNIEnvInit.SetSynchronizationContext()
+		}
+
 		static Context? _context;
 		public static Context Context {
 			get {

src/Mono.Android/Android.Runtime/JNIEnvInit.cs

@@ -86,6 +86,7 @@ internal static void InitializeJniRuntime (JniRuntime runtime)
 		{
 			androidRuntime = runtime;
 			ValueManager = runtime.ValueManager;
+			SetSynchronizationContext ();
 		}
 
 		[UnmanagedCallersOnly]
@@ -136,8 +137,6 @@ internal static unsafe void Initialize (JnienvInitializeArgs* args)
 		[DllImport ("xamarin-app")]
 		static extern unsafe void xamarin_app_init (IntPtr env, delegate* unmanaged <int, int, int, IntPtr*, void> get_function_pointer);
 
-		// NOTE: prevents Android.App.Application static ctor from running
-		[MethodImpl (MethodImplOptions.NoInlining)]
 		static void SetSynchronizationContext () =>
 			SynchronizationContext.SetSynchronizationContext (Android.App.Application.SynchronizationContext);
 	}