Simplify handling of ANDROID_JACK_* variables

This concerns ANDROID_JACK_VM, ANDROID_JACK_VM_ARGS and
ANDROID_JACK_EXTRA_ARGS.

Change-Id: Iba75ecaaceb09ac9607a3aee41324d9f5ae4beef
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index bc6661e..085b5fc 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -107,9 +107,9 @@
 LOCAL_ADDITIONAL_JAVA_DIR:=
 LOCAL_ALLOW_UNDEFINED_SYMBOLS:=
 LOCAL_DX_FLAGS:=
-LOCAL_USE_JACK:=$(ANDROID_COMPILE_WITH_JACK)
-LOCAL_JACK_VM_ARGS :=
-LOCAL_JACK_EXTRA_ARGS :=
+LOCAL_USE_JACK:= $(ANDROID_COMPILE_WITH_JACK)
+LOCAL_JACK_VM_ARGS := $(DEFAULT_JACK_VM_ARGS)
+LOCAL_JACK_EXTRA_ARGS := $(DEFAULT_JACK_EXTRA_ARGS)
 LOCAL_JACK_FLAGS:=
 LOCAL_CERTIFICATE:=
 LOCAL_SDK_VERSION:=