Move image wallpaper rendering works to background thread.

Doing image wallpaper rendering works on main thread may cause other
animation in system ui janky, so move the drawing task of image
wallpaper to another thread. We also make draw low priority.

Bug: 135601062
Test: observe log and systrace
Change-Id: I7bda6a757cb63f97f6b03c4b035fedbf87f91c96