Also store reduced resolution screenshots

In order to speed up loading time when scrolling through it
in recents. They will be used in recents in the next CL. Also, we
use JPG instead as loading JPG is much faster than PNG.

Test: TaskSnapshotPersisterLoaderTest
Test: TaskSnapshotCacheTest
Bug: 34829962
Change-Id: I4c74b26969ae459bd3b1a42707011a49f425abd9