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/static_java_library.mk b/core/static_java_library.mk
index 343b949..db64a8d 100644
--- a/core/static_java_library.mk
+++ b/core/static_java_library.mk
@@ -199,7 +199,7 @@
 	# Note: Use "cp -n" to honor the resource overlay rules, if multiple res dirs exist.
 	$(hide) $(foreach res,$(PRIVATE_RESOURCE_DIR),cp -Rfn $(res)/* $(dir $@)aar/res;)
 	$(hide) cp $(PRIVATE_R_TXT) $(dir $@)aar/R.txt
-	$(hide) jar -cMf $@ \
+	$(hide) $(JAR) -cMf $@ \
 	  -C $(dir $@)aar .
 
 # Register the aar file.