Review `JNIEnv.RegisterJniNatives()` for allocations

[jonpryor]

When recently profiling Xamarin.Android application startup, we observed that lots of values were being allocated within JNIEnv.RegisterJniNatives()

We should review this method and anything it calls to see if there's anything we can do differently.

(Obvious "do differently" angle: https://github.com/xamarin/xamarin-android/projects/1)

VS bug #660391

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