Support MPEG4 and H263 FourCC types in Matroska files

Handles MPEG4 and H263 content with FourCC header in Matroska files
based on how these are handled in the AVI parser.

Also adding placehoders for DivX.

Change-Id: I38b2c2fd429b6e4810a7b118bf60823d502320ae
stagefright: Fix crash if PCM offload gets a null source

Change-Id: I4db08355fb132288cec2ba99ec4dcb77f1618add