Track size changes correctly for static wallpapers

Previous logic compared the surface size to the bitmap size to determine
whether to reload the bitmap. This was based on an assumption that the bitmap
would be created at the same sizea s the surface. However, the process of
how those sizes get determined is different for surfaces and wallpapers, causing
an occasional issue where the bitmap gets reloaded frequently, every time the wallpaper
is asked to redraw, even though it always gets recreated at the same size.

New logic checks previous surface dimensions against current surface dimensions to
determine whether the bitmap should be reloaded; we really only want to reload
it when the surface size changes.

Issue #7373200 pause when toggling between All Apps and Home screen; Home button stays illuminated for a long time

Change-Id: I108777b72bd42616ad7cf8274af1b3e6b2ed94e7