stagefright: httplive: Fix deadlock for low duration clips

PlaylistFetcher buffers up to 3 * target-duration bytes of data,
but if a stream is slow (ie due to bad network conditions), a
buffer threshold of 10s is used to resume playback. This results
in an indefinite freeze as PlaylistFetcher has stopped buffering
before this threshold. Reduce the 10s threshold to be more in-sync
with PlaylistFetcher's buffering size.

CRs-Fixed: 745835
Change-Id: Ife846e7c5b4f9645895873d08250c4bee0164972