libstagefright - Fix seek on audio having empty stss block

Ignore stss block for audio even if its present, all audio
sample are sync samples itself, self decodeable and playable.

Parsing this block for audio restricts audio seek to few entries
available in this block, sometimes 0, which is undesired.

CRs-Fixed: 664870
Change-Id: If3208d5c0557d3eaeddf875d6c6762bd870c439b