[Xamarin.Android.Build.Tasks] Null Reference Exceoption when getting android.jar

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=60324

If a android.jar file does NOT exist for a target platform we should
raise a normal error. But we currently throw a null reference
exception.

`TryGetPlatformDirectoryFromApiLevel` can return null, this is by
design. So we need to handle that in the places where its used.
And return a reasonable error message if we do get a null back.

Author: dellis1972

src/Xamarin.Android.Build.Tasks/Tasks/GetJavaPlatformJar.cs

@@ -69,7 +69,8 @@ public override bool Execute ()
 			}
 
 			platform            = GetTargetSdkVersion (platform, target_sdk);
-			JavaPlatformJarPath = Path.Combine (MonoAndroidHelper.AndroidSdk.TryGetPlatformDirectoryFromApiLevel (platform, MonoAndroidHelper.SupportedVersions), "android.jar");
+			var platformPath = MonoAndroidHelper.AndroidSdk.TryGetPlatformDirectoryFromApiLevel (platform, MonoAndroidHelper.SupportedVersions);
+			JavaPlatformJarPath = Path.Combine (platformPath ?? MonoAndroidHelper.AndroidSdk.GetPlatformDirectoryFromId (platform), "android.jar");
 
 			if (!File.Exists (JavaPlatformJarPath)) {
 				Log.LogError ("Could not find android.jar for API Level {0}. " +

src/Xamarin.Android.Build.Tasks/Tasks/JarToXml.cs

@@ -68,7 +68,8 @@ public override bool Execute ()
 			}
 
 			// Ensure that the user has the platform they are targeting installed
-			var jarpath = Path.Combine (MonoAndroidHelper.AndroidSdk.TryGetPlatformDirectoryFromApiLevel (AndroidApiLevel, MonoAndroidHelper.SupportedVersions), "android.jar");
+			var platformPath = MonoAndroidHelper.AndroidSdk.TryGetPlatformDirectoryFromApiLevel (AndroidApiLevel, MonoAndroidHelper.SupportedVersions);
+			var jarpath = Path.Combine (platformPath ?? MonoAndroidHelper.AndroidSdk.GetPlatformDirectoryFromId (AndroidApiLevel), "android.jar");
 
 			if (!File.Exists (jarpath)) {
 				Log.LogError ("Could not find android.jar for API Level {0}.  This means the Android SDK platform for API Level {0} is not installed.  Either install it in the Android SDK Manager, or change your Android Bindings project to target an API version that is installed. ({1} missing.)", AndroidApiLevel, jarpath);