Discard hidden files when rounding up resource.

Aapt will discard the hidden files anyway.
With this change, we can avoid unnecessary rebuild if a module has only
hidden files in its resource dir.

Change-Id: Iaf0a13e038cca5fb60012c550cc90f7abbe967e1

core/definitions.mk

@@ -312,7 +312,7 @@ endef
 
 define find-subdir-assets
 $(if $(1),$(patsubst ./%,%, \
-	$(shell if [ -d $(1) ] ; then cd $(1) ; find ./ -type f -and -not -type l ; fi)), \
+	$(shell if [ -d $(1) ] ; then cd $(1) ; find ./ -not -name '.*' -and -type f -and -not -type l ; fi)), \
 	$(warning Empty argument supplied to find-subdir-assets) \
 )
 endef