Fix http://b/2126318 (density test fails due to wrong-size asset stripping)

The density test expects all resources of all densities to be included, but specifying a density for a particular device build kills that. We therefore add all the required densities explicitly to the Makefile to compensate.
diff --git a/tests/AndroidTests/Android.mk b/tests/AndroidTests/Android.mk
index f5e49d7..ced796a 100644
--- a/tests/AndroidTests/Android.mk
+++ b/tests/AndroidTests/Android.mk
@@ -8,7 +8,7 @@
 LOCAL_STATIC_JAVA_LIBRARIES := googlelogin-client
 
 # Resource unit tests use a private locale
-LOCAL_AAPT_FLAGS = -c xx_YY -c cs
+LOCAL_AAPT_FLAGS = -c xx_YY -c cs -c 160dpi -c 32dpi -c 240dpi
 
 LOCAL_SRC_FILES := \
 	$(call all-subdir-java-files) \