New PRODUCT_AAPT_PREF_CONFIG.

To use the new aapt --preferred-configurations option.  For use with Prime
to be able to strip everything but xhdpi density bitmaps selectively,
not stripping when this would result in no data for the resource.

Change-Id: I4e1012929b8f9b0b1e79c06496647f69661ba1f2