HLS: QCom enhancements

This commit consists of:

http://go/pag/c/188753 Add NULL check for empty playlist
http://go/pag/c/188754 Fix deadlock for low duration clips
http://go/pag/c/188757 Create a copy of last enqueued metadata
http://go/pag/c/188755 Propagate target duration to LiveSession
http://go/pag/c/188762 Decouple block size from bandwidth estimate
http://go/pag/c/188756 Reduce memcpy calls for chunked content
http://go/pag/c/188758 Dont resume if we have almost fetched till stop time

Bug: 18821145
Change-Id: I7fd650999c6c50bbadffd65adee9020e669dfe62