The fundamentals of an Android SDL2 build.

How to build:

1) manually build GENERATED_FILES as specified in the main Makefile
   (TODO: find a way to trigger building those from here).
2) ndk-build
3) ant (debug | release)

This uses our pre-existing contribs for support libraries - no
monolithic contrib with all the dependencies.

Currently this can build and start, but it crashes on trying to leave the
"press any key to begin" screen. TODO: figure out why, possibly by
figuring out how to get a useful stack trace on Android.

Other things to fix:
- Get a clean way to invoke this build from the main Makefile.
- Don't hard-code the list of source files and CFLAGS, possibly related
  to the above.
- The old SDL build used SDL_Mixer to provide sound. We could do an
  SDL2_Mixer contrib if we wanted to do that.