Mpeg4Extractor: Fix flac parsing in mp4

Flac in mp4 can have multiple metadata tags.
Added handling for the same.

Bug: 227296847
Test: stagefright -ao flac.mp4
Test: atest ExtractorUnitTest -- --enable-module-dynamic-download=true
Test: atest android.mediav2.cts

Change-Id: I3b8c4d374073c5dfd32f5f1b15d162103a7d6f5c
(cherry picked from commit 2039d4fb)
Merged-In: I3b8c4d374073c5dfd32f5f1b15d162103a7d6f5c