[Mono.Android] fix "replaceable" objects in ManagedValueManager

The following test is failing on NativeAOT as well as any case we'd
use `ManagedValueManager`:

    [Test]
    public void JnienvCreateInstance_RegistersMultipleInstances ()
    {
        using (var adapter = new CreateInstance_OverrideAbsListView_Adapter (Application.Context)) {

            var intermediate  = CreateInstance_OverrideAbsListView_Adapter.Intermediate;
            var registered    = Java.Lang.Object.GetObject<CreateInstance_OverrideAbsListView_Adapter>(adapter.Handle, JniHandleOwnership.DoNotTransfer);

            Assert.AreNotSame (adapter, intermediate); // Passes
            Assert.AreSame (adapter, registered);      // Fails!
        }
    }

With the assertion:

    Expected: same as <com.xamarin.android.runtimetests.CreateInstance_OverrideAbsListView_Adapter{cbd0e5a V.ED.VC.. ......I. 0,0-0,0}>
    But was:  <com.xamarin.android.runtimetests.CreateInstance_OverrideAbsListView_Adapter{cbd0e5a V.ED.VC.. ......I. 0,0-0,0}>

The second assertion fails because `registered` is the same instance
as `intermediate`. In this example, this is a code path where
`intermediate` should be "replaced" with `adapter`.

After lots of debugging, I found the problem are these lines in the
`ManagedValueManager.AddPeer()` method:

    var o = PeekPeer (value.PeerReference);
    if (o != null)
        return;

If we `PeekPeer()` in the middle of `AddPeer()` and a type is
"replaceable", it would find an instance and not replace it! I did not
find equivalent code in `AndroidValueManager.AddPeer()`, which is what
is used in Mono & production today.

With these lines removed, the test passes. I will look if we should
also update these lines in dotnet/java-interop in a future PR.

Author: jonathanpeppers

src/Mono.Android/Microsoft.Android.Runtime/ManagedValueManager.cs

@@ -67,9 +67,6 @@ public override void AddPeer (IJavaPeerable value)
 		var r = value.PeerReference;
 		if (!r.IsValid)
 			throw new ObjectDisposedException (value.GetType ().FullName);
-		var o = PeekPeer (value.PeerReference);
-		if (o != null)
-			return;
 
 		if (r.Type != JniObjectReferenceType.Global) {
 			value.SetPeerReference (r.NewGlobalRef ());