andrewlewis a4e7274cca Update audio extension build configurations
- 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
2019-11-05 16:16:38 +00:00
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