Extract crop hint rect from source wallpaper image

Setting the wallpaper is still synchronous: the caller blocks until any
backend cropping/manipulation has completed.  There is a timeout (currently
30 seconds) on that to avoid wedging the caller arbitrarily.

Bug 25454501

Change-Id: Idca2fe1b10e4fa34d6d54865903d9a1b9e305e3c