stagefright: HLS: Increase granularity of segment download

Current HLS logic requires downloading an entire segment before
bandwidth estimation or any other scheduled message can be
processed which hurts the effectiveness of the downswitch monitor.
If a large clip is being downloaded and the bandwidth drops
suddenly, playback can hang until download of the large clip
completes regardless of the downswitch monitor's status.

Expand onDownloadNext into a series of onDownloadBlock messages
to allow increased potential for message interleaving, but ensure
that segment downloads themselves are not interleaved as
LiveSession reuses a single http connection.

CRs-Fixed: 759591 771954
Change-Id: I6a9ba27e22cc4e04419ce0128ad0193edac9c42b