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