Use Q=100 JPEG instead of PNG for wallpaper display

The quality difference is minimal, but the time to process the
wallpaper is an order of magnitude lower for JPEG.  The source
imagery is still being preserved in its original format and
resolution; this affects only the scaled/cropped version used
on-screen.

Bug 28672266

Change-Id: I193854348ffb7eeb9e45dc08b8ef7173ea75c240