Clarify AC-4 mimetype is sync frame transport format

This enforces consistency with the AudioFormat ENCODING_AC4_L4 which
specifies AC-4 sync frames. AudioTrack AC-4 playback requires sync
frames, so harmonize both audio and media framework to expect and
require AC-4 to be in sync frame transport format.

MediaExtractors, including those for MP4 that packetize AC-4 raw
frames, are required to produce AC-4 sync frames. Similarly,
MP4 MediaMuxers must extract AC-4 raw frames from AC-4 sync frames
before packetizing.

Flag: EXEMPT doc only
Test: N/A doc only
Bug: 422475939
Change-Id: Ie9f9427aec788a8ea1979cfa13d23b277a6efbdb