Fix handling of ID3v2.4 extended header

If an optional ID3v2.4 extended header was present, it would be
treated as a frame, causing parse errors. Fix by doing ID3v2.4
per-frame unsynchronization after skipping over the extended
header.

Bug: 154357105
Bug: 151448144
Test: CTS, manual w/ temporary additional logging.
Change-Id: Iaeb8cab866b98dcf97f81d3ade4a1c8d26add3a5