First pass at NativeActivity.

This is a rough sketch of the new pure-native API, which you can
use through a NativeActivity in your manifest (no Java code in
the .apk needed!).

Intentionally no docs yet, the API is still being seriously
messed with.  But it works.

Change-Id: I0e916d58a0d159ecaf3689e41834eb8dc681c0c0