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/Makefile b/core/Makefile
index 9aae8ca..8dbd5ed 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1093,7 +1093,7 @@
 	@echo "DumpPublicKey: $@ <= $(PRIVATE_OTA_PUBLIC_KEYS) $(extra_keys)"
 	@rm -rf $@
 	@mkdir -p $(dir $@)
-	java -jar $(DUMPKEY_JAR) $(PRIVATE_OTA_PUBLIC_KEYS) $(extra_keys) > $@
+	$(JAVA) -jar $(DUMPKEY_JAR) $(PRIVATE_OTA_PUBLIC_KEYS) $(extra_keys) > $@
 
 RECOVERYIMAGE_ID_FILE := $(PRODUCT_OUT)/recovery.id