Switch to FFMpeg ExoPlayer extension

[OxygenCobalt]

Switch to the FFMpeg extension for ExoPlayer in place of the FLAC extension.

This would:

• Enable support for ALAC on all versions (See M4A files have no sound #337)
• Retain support for FLAC below Android 8
• If I patch ExoPlayer, allow the hypothetical addition of more obscure formats like WavPack and Monkey's Audio. This isn't likely though since it's hard to create first-class support for these formats elsewhere.