Notes:
- We can actually use a single Activity.java file for oculus/pico now
- We can unconditionally compile os_android.c on Android
- No need for including extra jars in build system
- Headset rendering is guaranteed synchronous now, no need to ref L
- Add an "android flavor" build setting to differentiate between oculus
and pico devices, since they both use OpenXR.
- Update the pico manifest to reflect their OpenXR sample
- Remove some OpenGL hacks that aren't necessary anymore
Add entrypoints, headset backend code, fill in the Activity, and
add various special cases to account for the asynchronous render loop,
lack of sRGB support, and OpenGL state resets.