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
diff --git a/core/product.mk b/core/product.mk
index 72cb2f7..0ab9ae0 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -64,6 +64,7 @@
     PRODUCT_MODEL \
     PRODUCT_LOCALES \
     PRODUCT_AAPT_CONFIG \
+    PRODUCT_AAPT_PREF_CONFIG \
     PRODUCT_PACKAGES \
     PRODUCT_DEVICE \
     PRODUCT_MANUFACTURER \