This fixes build errors about not specifying either of the
"native-activity" or "game-activity" features.
The issue came about because the in-tree examples want to
build against the in-tree version of android-activity located
with a relative `path = ` but these particular examples
depend on Winit which would resolve a second implementation
of android-activity, via a github url - where Cargo will treat
them as completely different crates.
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