stagefright: Validate track formats passed to MPEG4Writer

When authoring the FourCC box, the framework asserts if provided
with an unsupported mime. Validate the mime type for video tracks
when they are first added. If the passed format is invalid, return
error immediately to avoid triggering the assertion later.

CRs-Fixed: 696137
Change-Id: Ie80372940a46038db73107101a2ae6fdecf72eaa