Fix bad JACK_PLUGIN variable

Changed to LOCAL_JACK_PLUGIN.

(cherry picked from commit 17924b136cb5bfc60d138e8c7e5c18a90b5965c3)

Test: Manually tested by some manual activations of the coverage plugin.
Change-Id: I1be4aaa502103fc308d35a5672dc4fe5900c4ebe
diff --git a/core/java.mk b/core/java.mk
index b74c37d..9199f7c 100644
--- a/core/java.mk
+++ b/core/java.mk
@@ -636,7 +636,7 @@
 
 ifeq ($(LOCAL_IS_STATIC_JAVA_LIBRARY),true)
 $(full_classes_jack): PRIVATE_JACK_PLUGIN_PATH := $(LOCAL_JACK_PLUGIN_PATH)
-$(full_classes_jack): PRIVATE_JACK_PLUGIN := $(JACK_PLUGIN)
+$(full_classes_jack): PRIVATE_JACK_PLUGIN := $(LOCAL_JACK_PLUGIN)
 $(full_classes_jack): $(jack_all_deps) $(LOCAL_JACK_PLUGIN_PATH) | setup-jack-server
 	@echo Building with Jack: $@
 	$(java-to-jack)
@@ -679,7 +679,7 @@
 endif #LOCAL_IS_STATIC_JAVA_LIBRARY
 
 $(noshrob_classes_jack): PRIVATE_JACK_PLUGIN_PATH := $(LOCAL_JACK_PLUGIN_PATH)
-$(noshrob_classes_jack): PRIVATE_JACK_PLUGIN := $(JACK_PLUGIN)
+$(noshrob_classes_jack): PRIVATE_JACK_PLUGIN := $(LOCAL_JACK_PLUGIN)
 $(noshrob_classes_jack): PRIVATE_JACK_INTERMEDIATES_DIR := $(intermediates.COMMON)/jack-noshrob-rsc
 $(noshrob_classes_jack): PRIVATE_JACK_PROGUARD_FLAGS :=
 $(noshrob_classes_jack): $(jack_all_deps) $(LOCAL_JACK_PLUGIN_PATH) | setup-jack-server