SDK / exoplayer

- Fix FLAC extension build (currently broken due to use of std::array,
  but fixed by migrating to NDK r20).
- Move opus and ffmpeg extensions to NDK r20. For ffmpeg, upgrade to
  release 4.2 which requires using libswresample and updates to the
  build script.

Issue: #6601
PiperOrigin-RevId: 277924119