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