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/package.mk b/core/package.mk
index 398e487..07cdbd0 100644
--- a/core/package.mk
+++ b/core/package.mk
@@ -227,6 +227,7 @@
 # add-assets-to-package looks at PRODUCT_AAPT_CONFIG, but this target
 # can't know anything about PRODUCT.  Clear it out just for this target.
 $(resource_export_package): PRIVATE_PRODUCT_AAPT_CONFIG :=
+$(resource_export_package): PRIVATE_PRODUCT_AAPT_PREF_CONFIG :=
 $(resource_export_package): $(all_res_assets) $(full_android_manifest) $(RenderScript_file_stamp) $(AAPT)
 	@echo "target Export Resources: $(PRIVATE_MODULE) ($@)"
 	$(create-empty-package)
@@ -343,8 +344,10 @@
 endif
 ifeq ($(LOCAL_AAPT_INCLUDE_ALL_RESOURCES),true)
     $(LOCAL_BUILT_MODULE): PRIVATE_PRODUCT_AAPT_CONFIG :=
+    $(LOCAL_BUILT_MODULE): PRIVATE_PRODUCT_AAPT_PREF_CONFIG :=
 else
     $(LOCAL_BUILT_MODULE): PRIVATE_PRODUCT_AAPT_CONFIG := $(PRODUCT_AAPT_CONFIG)
+    $(LOCAL_BUILT_MODULE): PRIVATE_PRODUCT_AAPT_PREF_CONFIG := $(PRODUCT_AAPT_PREF_CONFIG)
 endif
 $(LOCAL_BUILT_MODULE): $(all_res_assets) $(jni_shared_libraries) $(full_android_manifest)
 	@echo "target Package: $(PRIVATE_MODULE) ($@)"