stagefright: Validate track formats passed to MPEG4Writer

Validate the mime type for tracks to start with; and throw
an error for unsupported types rather than let apps re-mux/encode
the whole clip and assert towards the end while writing the header.
The writer asserts if provided with an unsupported mime when
authoring the fourcc, which is done at the end of muxing.

Bug: 17007397
Change-Id: Ie80372940a46038db73107101a2ae6fdecf72eaa