Provide make variables for java binaries

Replace java -> $(JAVA), etc. to prepare for moving java binaries
out of the path and using full paths to them.

Test: builds
Bug: 62956999
Change-Id: I5dfcd0d4e855ecde69cd127169e9545e59723d12
diff --git a/core/package_internal.mk b/core/package_internal.mk
index 6255ef1..8b3c375 100644
--- a/core/package_internal.mk
+++ b/core/package_internal.mk
@@ -355,7 +355,7 @@
 	@rm -rf $(PRIVATE_INTERMEDIATES) $(PRIVATE_SRC_OUT) && \
 	  mkdir -p $(PRIVATE_INTERMEDIATES) $(PRIVATE_SRC_OUT) \
 	      $(PRIVATE_XML_OUT) $(PRIVATE_RES_OUT) $(PRIVATE_ANNO_SRC_DIR)
-	$(hide) java -classpath $(DATA_BINDING_COMPILER) android.databinding.tool.MakeCopy \
+	$(hide) $(JAVA) -classpath $(DATA_BINDING_COMPILER) android.databinding.tool.MakeCopy \
 	  $(PRIVATE_MANIFEST) $(PRIVATE_SRC_OUT) $(PRIVATE_XML_OUT) $(PRIVATE_RES_OUT) $(PRIVATE_RES_IN)
 	$(hide) touch $@