[corlib] add InternalsVisibleTo for Mono.Android (#18040)

[corlib] add InternalsVisibleTo for Mono.Android (#18040)

* [corlib] add InternalsVisibleTo for Mono.Android

Context: https://github.com/xamarin/xamarin-android/pull/3991

In Xamarin.Android, we have a bit of System.Reflection usage on
startup:

https://github.com/xamarin/xamarin-android/blob/bbbe2ed36e6eeb7cce0905dcbfdaeaa30754070f/src/Mono.Android/Android.Runtime/UncaughtExceptionHandler.cs#L47-L67

These methods are in `mscorlib.dll` and `internal`. If we had
`[assembly: InternalsVisibleTo ("Mono.Android")]` we could avoid the
System.Reflection.

This is already done for `Xamarin.iOS`, `Xamarin.Mac`, etc. so
hopefully it's OK to add one more?

We could then, just call these methods directly:

* `System.Diagnostics.Debugger.Mono_UnhandledException`
* `AppDomain.CurrentDomain.DoUnhandledException`

The results are possibly a 100-250ms improvement to startup:

    Before:
    12-03 14:27:51.450  1876  1898 I ActivityManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +3s727ms
    12-03 14:27:57.310  1876  1898 I ActivityManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +3s723ms
    12-03 14:28:03.216  1876  1898 I ActivityManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +3s703ms
    12-03 14:28:09.054  1876  1898 I ActivityManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +3s679ms
    12-03 14:28:14.953  1876  1898 I ActivityManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +3s711ms
    After:
    12-04 09:13:18.248  1876  1898 I ActivityManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +3s608ms
    12-04 09:13:24.087  1876  1898 I ActivityManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +3s570ms
    12-04 09:13:29.997  1876  1898 I ActivityManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +3s566ms
    12-04 09:13:35.805  1876  1898 I ActivityManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +3s498ms
    12-04 09:13:41.670  1876  1898 I ActivityManager: Displayed Xamarin.Forms_Performance_Integration/xamarin.forms.performance.integration.MainActivity: +3s475ms

This was the Xamarin.Forms integration project in xamarin-android.
Running a `Debug` build on a HAXM x86 emulator on Windows.

https://github.com/xamarin/xamarin-android/tree/master/tests/Xamarin.Forms-Performance-Integration

* Bump API snapshot submodule