Improve DiskBasedCache initial scan performance

Using a BufferedInputStream significantly reduces the time it takes
to initialize the cache. On my Nexus 5 with ~330 files in cache init
time went from ~2s to ~0.5s with this change.

Change-Id: I3dd909e3235583306ff5943f69e3947a275bf968
Signed-off-by: Fabian Frank <fabian.frank.de@gmail.com>
Merge "Improve DiskBasedCache initial scan performance"
Merge branch 'master' into imageLoader_with_authToken