stagefright: Increase block size for download

  Since a single TCP/IP packet can contain up to 64k of data, increase
the block size in PlaylistFetcher from 2k to lcm(188, 1024) or 47k to
avoid inaccuracies in read timings due to up to a comparable 47 reads
from the same locally-cached packet instead of from the network

CRs-Fixed: 771954
Change-Id: I348dc1fc080b822e086729638cc0ff166a13016d