Always use low-res tasksnapshots for low ram devices

All snapshots are now stored using only the low resolution bitmaps
where all full size bitmaps are disabled to be written or loaded.

Bug: 62251652
Fixes: 63940837
Test: manual - open recents on low ram device to see if thumbnail is
there
Change-Id: I2128f0348cf71415721e73c730d3ed92e95d8144