Use PRIVATE_WARNINGS_ENABLE

Before this LOCA_WARNINGS_ENABLE never worked :-(

Change-Id: I129a0bbbbb68e4179454b0827b0b0d18d6de67a0
diff --git a/core/java.mk b/core/java.mk
index cc498af..13906f7 100644
--- a/core/java.mk
+++ b/core/java.mk
@@ -258,6 +258,7 @@
 # Run build/tools/java-layers.py for more details.
 layers_file := $(addprefix $(LOCAL_PATH)/, $(LOCAL_JAVA_LAYERS_FILE))
 $(full_classes_compiled_jar): PRIVATE_JAVA_LAYERS_FILE := $(layers_file)
+$(full_classes_compiled_jar): PRIVATE_WARNINGS_ENABLE := $(LOCAL_WARNINGS_ENABLE)
 
 # Compile the java files to a .jar file.
 # This intentionally depends on java_sources, not all_java_sources.