[One .NET] call SynchronizationContext.SetSynchronizationContext() on startup (#5243)

Fixes: #4655

The following test is failing under .NET 5+ because
`SynchronizationContext.Current` returns `null`:

	[Test]
	public void SynchronizationContext_Is_ThreadingSynchronizationContextCurrent ()
	{
	    bool same = false;
	    Application.SynchronizationContext.Send (_ => {
	            var c = System.Threading.SynchronizationContext.Current;
	            same = object.ReferenceEquals (c, Application.SynchronizationContext);
	    }, null);
	    Assert.IsTrue (same);
	}

Following how this works in current Xamarin.Android / Mono:

  * https://github.com/mono/mono/blob/169a842f4e913608b17a1e5f38bcb635063b8dc8/mcs/class/referencesource/mscorlib/system/threading/synchronizationcontext.cs#L297
  * https://github.com/mono/mono/blob/d6f0c25d34da7512639346828ef368ff65a25003/mcs/class/corlib/System/AndroidPlatform.cs#L37-L53
  * https://github.com/xamarin/xamarin-android/blob/master/src/Mono.Android/Android.Runtime/AndroidEnvironment.cs#L289-L302

dotnet/runtime won't make the call to
`AndroidEvironment.GetDefaultSyncContext()`, as the same pattern is
not in place for other platforms.

The suggested alternative is to call
`SynchronizationContext.SetSynchronizationContext()` on startup.
We should be able to call `Application.SynchronizationContext`
directly to do this.

After these changes, the original code example works as expected:

	Application.SynchronizationContext.Send (_ => {
	    var c = System.Threading.SynchronizationContext.Current;
	    button.Text +=  " 1:" + c?.ToString();
	    var same = object.ReferenceEquals (c, Application.SynchronizationContext);
	    button.Text +=  " 2:" + Application.SynchronizationContext?.ToString();
	    button.Text +=  " 3:" + same;
	}, null);

Author: jonathanpeppers

src/Mono.Android/Android.Runtime/JNIEnv.cs

@@ -192,6 +192,10 @@ internal static unsafe void Initialize (JnienvInitializeArgs* args)
 				}
 			}
 
+#if !MONOANDROID1_0
+			SynchronizationContext.SetSynchronizationContext (Android.App.Application.SynchronizationContext);
+#endif
+
 			if (logTiming) {
 				monodroid_timing_stop (total_timing_sequence, "JNIEnv.Initialize end");
 			}