Add product variable PRODUCT_AAPT_PREBUILT_DPI

We use search LOCAL_DPI_VARIANTS in the list of
"$(PRODUCT_AAPT_PREF_CONFIG) $(PRODUCT_AAPT_PREBUILT_DPI)"
and the first takes precedence.
That way if we don't have a best match, we fall back to the second best,
the way how it worked with PRODUCT_AAPT_CONFIG previously.

Bug: 18388705
Change-Id: I8bd646c52215c65cc6e38c728857af9b64d13469
diff --git a/core/product.mk b/core/product.mk
index ed906cb..f0fc921 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -70,6 +70,7 @@
     PRODUCT_LOCALES \
     PRODUCT_AAPT_CONFIG \
     PRODUCT_AAPT_PREF_CONFIG \
+    PRODUCT_AAPT_PREBUILT_DPI \
     PRODUCT_PACKAGES \
     PRODUCT_PACKAGES_DEBUG \
     PRODUCT_PACKAGES_ENG \