Define GIDs for internal media file types.

Currently, all internal media storage files (under /data/media) are
given the same "media_rw" GID.  To support an upcoming optimization
to disk usage calculation, this CL defines three new GIDs that can be
used to quickly identify common types: audio, video, and images.

Test: builds
Bug: 27948817
Change-Id: I8548459c3d3ccbf7d822431c6e2ebd1c2e309d70