stagefright: allow targets to pass real dimensions to the decoder

This changeset adds TARGET_SF_NEEDS_REAL_DIMENSIONS. When set to true,
it will make stagefright pass the real video dimensions to the decoder.
When not set or set to false, it will fallback to the old behaviour,
that is, forcing dimensions to 352x288

This fixes video thumbnailing on zeppelin

Change-Id: I745356de4a0182e048b37318f556ae05c64a6f31