Initial support for subtitles and lyrics.

Ideally audio and video files would embed information like subtitles
and lyrics directly into the container format, but alas the world
hasn't completely moved to Matroska yet.

Thus we're willing to recognize these side-car files and let them be
visible to apps that have gained the "Audio" or "Video" permissions.

Bug: 132359020, 143549606
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: I07513c48e245a00c0288a4b0fbee322cb02f49a9