ImageCache: tweak configuration

The current configuration was very conservative for recent
devices, even if they are considered "Low-End".

Push the limits a bit higher to benefit better from caching.
 - Allow using 50% of device memory (was 25%)
 - Increase disk cache to 50 MB (was 10 MB)

Change-Id: Idff956de6c7181f8b529ef1e84f183b370192735
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>