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/prebuilt_internal.mk b/core/prebuilt_internal.mk
index 4095ab7..c31a07e 100644
--- a/core/prebuilt_internal.mk
+++ b/core/prebuilt_internal.mk
@@ -481,7 +481,7 @@
 $(intermediates.COMMON)/classes.jack : PRIVATE_JACK_FLAGS:=$(LOCAL_JACK_FLAGS)
 $(intermediates.COMMON)/classes.jack : PRIVATE_JACK_MIN_SDK_VERSION := $(if $(strip $(LOCAL_MIN_SDK_VERSION)),$(LOCAL_MIN_SDK_VERSION),1)
 $(intermediates.COMMON)/classes.jack : PRIVATE_JACK_PLUGIN_PATH := $(LOCAL_JACK_PLUGIN_PATH)
-$(intermediates.COMMON)/classes.jack : PRIVATE_JACK_PLUGIN := $(JACK_PLUGIN)
+$(intermediates.COMMON)/classes.jack : PRIVATE_JACK_PLUGIN := $(LOCAL_JACK_PLUGIN)
 $(intermediates.COMMON)/classes.jack : $(LOCAL_JACK_PLUGIN_PATH) $(my_src_jar) \
         $(LOCAL_ADDITIONAL_DEPENDENCIES) $(JACK_DEFAULT_ARGS) $(JACK) \
         | setup-jack-server