Engage Jack incremental on demand

Each module may engage Jack incremental with

LOCAL_JACK_ENABLED := incremental

Include renaming of LOCAL_USE_JACK to LOCAL_JACK_ENABLED that is
now accepting 3 values "disabled", "full" and "incremental".

Change-Id: Icbff275b397bee36b29312e821f3e8d45f83fbcc
diff --git a/core/java_library.mk b/core/java_library.mk
index 8641aad..5ba8267 100644
--- a/core/java_library.mk
+++ b/core/java_library.mk
@@ -69,13 +69,13 @@
 $(common_javalib.jar): PRIVATE_DONT_DELETE_JAR_DIRS := $(LOCAL_DONT_DELETE_JAR_DIRS)
 $(common_javalib.jar) : $(built_dex) $(java_resource_sources)
 	@echo "target Jar: $(PRIVATE_MODULE) ($@)"
-ifeq ($(LOCAL_USE_JACK),true)
+ifdef LOCAL_JACK_ENABLED
 	$(create-empty-package)
 else
 	$(call initialize-package-file,$(PRIVATE_SOURCE_ARCHIVE),$@)
 endif
 	$(add-dex-to-package)
-ifeq ($(LOCAL_USE_JACK),true)
+ifdef LOCAL_JACK_ENABLED
 	$(add-carried-jack-resources)
 endif