MPEG4Writer: Add support for AV1 muxing

Add support for AV1 muxing in the MPEG4Writer. Most of the support
is already in place. We would have to allow the AV1 mime type and
write the AV1 codec configuration box (av1C).

Note that this CL only supports muxing of AV1 streams where the
codec specific data is passed in via the MediaFormat. Follow-up
CLs will address the case of muxing AV1 streams without explicitly
supplied codec specific data.

Test: New cts tests in CtsMediaMuxerTestCases pass
Bug: 206014067
Change-Id: I20fa2de1e7aebfe81d2fbc71dfbd252dea07e330