libstagefright: Ensure cache prefetch in underrun

NuCachedSource2::onFetch() will stop prefetching
if the total cache size exceeds highwater mark,
rather than considering the size of unaccessed data
for the calculation. If AwesomePlayer pauses due to
underrun, and NuCachedSource2 stop fetching, this
can lead to indefinite pause as AwesomePlayer will
use the unaccessed data for calculating buffered
duration while NuCachedSource2 uses the total cached
size

Added change to ensure cache prefetch in underrun so
that playback does not pause indefinitely

CRs-Fixed: 760796
Change-Id: I8124e04b18a6e200b59c9cf7d2a912a43e147db1