bug 50733: Fix OOM error on low-memory devices

LRU bitmap cache ate too much memory on low-memory devices like the
old Nexus One and some of our testers' devices.

Removed the in-memory cache in favor of letting Volley use its disk-
based cache. This may mean slightly more overhead pulling images
out of cache, but saves a few megabytes of RAM which makes a big
difference when the per-process limit is only 24MB...