stagefright: httplive: Enable dumping of fetched segments

If SAVE_BACKUPS is enabled, create a numbered backup file for each
new fetchFile() connection and archive all received bytes within
/data/misc/media. Increase the number for each new connection
within a playback session to aid in debugging switching use-cases.

Also immediately flush all data after each readAt() to allow
debugging crashes/asserts without loss of retrieved data.

CRs-Fixed: 771954
Change-Id: I38cb9653ffff8e8c6253a8caa0a7ee4a0bc7816f