Use a less freqent dummy file name: zipdummy

Previously if user has a directory with name dummy in the root of the
source tree, "zip -qd package.apk dummy" fails with:
"zip error: Nothing to do!".
This change mitigates the error.

Change-Id: I642e3bf0378e5b9911a068ecb72f795b3e92f1fe
diff --git a/core/Makefile b/core/Makefile
index e52fbc8..0de717a 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1536,10 +1536,10 @@
 		  unobfuscated_jars=$${dict_files//proguard_dictionary/classes.jar}; \
 		  zip -q $@ $$dict_files $$unobfuscated_jars; \
 		else \
-		  touch $(dir $@)/dummy; \
-		  (cd $(dir $@) && zip -q $(notdir $@) dummy); \
-		  zip -qd $@ dummy; \
-		  rm $(dir $@)/dummy; \
+		  touch $(dir $@)/zipdummy; \
+		  (cd $(dir $@) && zip -q $(notdir $@) zipdummy); \
+		  zip -qd $@ zipdummy; \
+		  rm $(dir $@)/zipdummy; \
 		fi
 
 endif # TARGET_BUILD_APPS