Force the wallpaper to reload when dimensions change

Currently it's possible for the home application to suggest new
wallpaper dimensions and the WallpaperService to request the bitmap
between when the new dimensions have been propagated and the old
bitmap has been forgotten. This leads to the WallpaperService
drawing a Bitmap with the old dimensions into a Surface with the new
dimensions.

By forcing the WallpaperManager to forget the old Bitmap immediately
before we reload it, we can ensure that we always have a Bitmap of
the correct size.

Bug: 10853302
Change-Id: I298ac5f3f8bcde54eeb1e45d21bf2ba3cbb618c9