stagefright: httplive: Reduce memcpy calls for chunked content

Streams using http chunking will not report the segment's total
content-length. In this case, a 64k buffer is allocated and is
increased by 32k each time the buffer is filled again. For high
bitrate content, this can lead to a large number of copies that
affect the HLS framework delay. Increase fetchFile buffer size
exponentially by 50% or at least 32k instead of by 32k each time
to reduce the number of memcpy calls.

Example for a chunked 6 MB 1080p segment (ie ~3s):
Adding 32k:
190 copies with 572.97 MB copied

Increasing by 50%:
12 copies with 16.09 MB copied

CRs-Fixed: 745835
Change-Id: Iedf0e4437e96026a58d50bce2660f85ac90d0ada