Enable xhdpi drawables while keeping mdpi as primary source

Should prevent mdpi device breakage as features are added that require
drawables, but do not include mdpi drawables, as appears to be the case
going forward with 4.2.2. Most recent example was Custom Nav Ring. This
change fixes the problem on tf101, tf300t, otter, etc.

Change-Id: Ic8a734d5e7b823de153b94eb21e71bdd5a5480e0