media: Add metadata track support to MPEG4Writer.

1. Remove the restriction that MPEG4Writer could only write at most one video
track and one audio track.
2. Allow muxing mutiple metadata tracks into the file. Did not put a limit on
each sample's metadata size now.
3. metadata mime format must start from prefix "application/".
4. metadata's format/layout is defined by the application.
5. Generated MP4 file use TextMetaDataSampleEntry defined in section 12.3.3.2
of the ISOBMFF to signal the metadata's mime format.

Test: Manual test.

Bug: 28536560
Change-Id: I97829e235e598e35fcad31f948eea5943f8c4fd5