Pass device characteristic information through to aapt.

A build configuration can define PRODUCT_CHARACTERISTICS, which is
a comma-separated list of device characteristics, for example
"nosdcard,grayscale".  This will be passed through to aapt with
the --product flag.

Change-Id: I902c034b2f2824dbd2d33449308ee08beeb09551
diff --git a/core/product.mk b/core/product.mk
index 7594f6f..4a0ee01 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -68,6 +68,7 @@
     PRODUCT_MANUFACTURER \
     PRODUCT_BRAND \
     PRODUCT_PROPERTY_OVERRIDES \
+    PRODUCT_CHARACTERISTICS \
     PRODUCT_COPY_FILES \
     PRODUCT_OTA_PUBLIC_KEYS \
     PRODUCT_PACKAGE_OVERLAYS \