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/cleanbuild.mk b/core/cleanbuild.mk
index 2ac572f..e6db5cb 100644
--- a/core/cleanbuild.mk
+++ b/core/cleanbuild.mk
@@ -118,7 +118,7 @@
 endif
 
 # A change in the list of aapt configs warrants an installclean, too.
-aapt_config_list := $(strip $(PRODUCT_AAPT_CONFIG))
+aapt_config_list := $(strip $(PRODUCT_AAPT_CONFIG) $(PRODUCT_AAPT_PREF_CONFIG))
 
 current_build_config := \
     $(TARGET_PRODUCT)-$(TARGET_BUILD_VARIANT)$(building_sdk)-{$(aapt_config_list)}