"native-activity" builds were recently broken by bb8eeb705c which
this patch fixes.
The na-mainloop example has also been updated to verify this by
reducing the fake "render" timeout and also triggering a fake
render when an InputAvailable event is received.
Fixes: #12
android_main() now takes an `app: AndroidApp` argument and has an
extern "Rust" ABI instead of "C" ABI.
This should help make it possible to run multiple native Activities in a
single process.
Robert Bragg