MPEG4Extractor: ensure kKeyTrackID exists before creating an MPEG4Source as track.

GenericSource: return error when no track exists.

SampleIterator: make sure mSamplesPerChunk is not zero before using it as divisor.

Bug: 21657957
Bug: 23705695
Bug: 22802344
Bug: 28799341
Change-Id: I7664992ade90b935d3f255dcd43ecc2898f30b04
(cherry picked from commit 0386c91b8a910a134e5898ffa924c1b6c7560b13)
