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.
This leaves the InputBuffer abstraction as an implementation detail
and just exposes an API like this for processing input:
app.input_events(|event| {
// handle event
});
The main consideration here was to have an API that could also be
supported via NativeActivity to keep open the possibility of a standard
Android 'glue' API.
Robert Bragg