[Xamarin.Android.Build.Tasks] default proguard rules for BroadcastReceiver

[jonathanpeppers]

Fixes: #4110

A crash was reported when using either Proguard/R8:

java.lang.RuntimeException: Unable to get provider mono.MonoRuntimeProvider_1: java.lang.ClassNotFoundException: Didn't find class "mono.MonoRuntimeProvider_1" on path: DexPathList[[zip file "/data/app/myApp-61Sw26LCrQQYfOvCs2GsDw==/base.apk"],nativeLibraryDirectories=[/data/app/myapp-61Sw26LCrQQYfOvCs2GsDw==/lib/arm64, /data/app/myapp-61Sw26LCrQQYfOvCs2GsDw==/base.apk!/lib/arm64-v8a, /system/lib64, /system/product/lib64]]
    at android.app.ActivityThread.installProvider(ActivityThread.java:7152)
    at android.app.ActivityThread.installContentProviders(ActivityThread.java:6630)
    at android.app.ActivityThread.handleBindApplication(ActivityThread.java:6525)
    at android.app.ActivityThread.access$1400(ActivityThread.java:220)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1883)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loop(Looper.java:224)
    at android.app.ActivityThread.main(ActivityThread.java:7520)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
    Caused by: java.lang.ClassNotFoundException: Didn't find class "mono.MonoRuntimeProvider_1" on path: DexPathList[[zip file "/data/app/myapp-61Sw26LCrQQYfOvCs2GsDw==/base.apk"],nativeLibraryDirectories=[/data/app/myapp-61Sw26LCrQQYfOvCs2GsDw==/lib/arm64, /data/app/myapp-61Sw26LCrQQYfOvCs2GsDw==/base.apk!/lib/arm64-v8a, /system/lib64, /system/product/lib64]]
    at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:230)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:379)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:312)
    at android.app.AppComponentFactory.instantiateProvider(AppComponentFactory.java:147)
    at android.app.ActivityThread.installProvider(ActivityThread.java:7136)

This could be reproduced in a new project, just by enabling a Code
Shrinker and adding a BroadcastReceiver:

[BroadcastReceiver(Process = ":remote", Name = "foo.MyReceiver")]
public class MyReceiver : BroadcastReceiver
{
    public override void OnReceive(Context context, Intent intent) { }
}

This generated a file in:

obj\Release\100\android\src\mono\MonoRuntimeProvider_1.java

We have no default proguard rule in proguard_xamarin.cfg that would
cover inclusion of this Java class for ProGuard or R8.

We can simply add a trailing * to the Java class name of an existing
rule:

-keep class mono.MonoRuntimeProvider* { *; <init>(...); }

I also added a test for this scenario that will verify ProGuard & R8.