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/definitions.mk b/core/definitions.mk
index 1afc0f4..1a7cf4b 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -1559,6 +1559,7 @@
 define add-assets-to-package
 $(hide) $(AAPT) package -u $(PRIVATE_AAPT_FLAGS) \
     $(addprefix -c , $(PRIVATE_PRODUCT_AAPT_CONFIG)) \
+    $(addprefix --preferred-configurations , $(PRIVATE_PRODUCT_AAPT_PREF_CONFIG)) \
     $(addprefix -M , $(PRIVATE_ANDROID_MANIFEST)) \
     $(addprefix -S , $(PRIVATE_RESOURCE_DIR)) \
     $(addprefix -A , $(PRIVATE_ASSET_DIR)) \