StagefrightMetadataRetriever - fixed source start assertion failure on MPEG4Source

 - OMXCodec calls start on MPEG4Source
 - in occasional cases during thumbnail generation when buffers cannot be allocated
   by OMXCodec, init() will fail along with start() with MPEG4Source still started.
   StagefrightMetadataRetriever will then attempt to start OMXCodec again which calls
   start on MPEG4Source, causing mStarted assertion failure in MPEG4Source start()
 - this patch calls stop on MPEG4Source in case when decoder->start() fails by calling
   destructor of MPEG4Source (which stops the source safely) and remaking the source
   using the getTrack() call in Extractor

CRs-Fixed: 326063, 325461, 323538
Change-Id: Ib16444f6d3a0589fdb26e79d8323ae1edf90a5a1