MPEG4Extractor: Read out the Metadata track correctly.

The metadata track before API29 is not compliant with the standard. This
CL adds the support for reading out the mime_format correctly from compliant
and non-compliant metadata track.

Bug:  117950811
Test: CTS test.
Change-Id: I9a7033549962711b625f5a008a22930691ab3e5e
1 file changed