Gallery2: fix memory leak and reduce memory use

Clear references to FilterShowActivity and reset singleton MasterImage
when exiting FilterShowActivity. Wait for saving task to complete before
exiting FilterShowActivity to resolve issue of re-entering activity
while previous image save task is still running. Do not upscale border
assets to reduce memory footprint.

Change-Id: If17479db2ae78955c15e95f72588e32019177bb9
CRs-Fixed: 929338