WallpaperColors hint computation

Public WallpaperColors surface should not compute hints, malicious apps
might use theme inversion to slow down the system.

Change-Id: I9d14259e433b1047d2512b3d994524dca5e8531a
Fixes: 69532159
Test: set white wallpaper, black wallpaper, observe theme changing
Test: runtest -x services/tests/servicestests/src/com/android/server/wallpaper/WallpaperServiceTests.java