Upscale images to fill screen for slideshow

Bug: 7110382
This fixes the slideshow mode on high-resolution
displays. Note that this will scale up screennails
to fill the screen even if they are fairly low quality.
The resolution to 7232758 will set the size of the
screennails appropriately based on screen size.

Change-Id: I7b5367c2fce1d10f6d5e70c1239d893f54236b10