Only rescale wallpaper if its > display height

WallpaperPicker assigns wallpapers, this code caches them.
The assigned wallpaper is ~always slightly smaller than the display size
There's no need to scale _up_ the assigned wallpaper, it will only
permanently introduce scaling artifacts while increasing the amount of
storage required to store the wallpaper.
Most importantly, it creates an inconsistency between the colors
WallpaperColors.fromBitmap returns, and the WallpaperColors returned
by the system from the cached copy. This causes a large problem for
dynamic color, as it means the colors displayed in Wallpaper Picker
will differ from the colors actually extracted by the system.

Bug: 189931209
Test: Test tons and tons of wallpapers over a couple days.
Change-Id: Ifefd3e56df57f55cead985c3f182db2de10dc769