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/static_java_library.mk b/core/static_java_library.mk
index c692f9f..c5d1b46 100644
--- a/core/static_java_library.mk
+++ b/core/static_java_library.mk
@@ -59,12 +59,12 @@
 
 LOCAL_PROGUARD_FLAGS := $(addprefix -include ,$(proguard_options_file)) $(LOCAL_PROGUARD_FLAGS)
 
-ifeq ($(LOCAL_USE_JACK),true)
+ifdef LOCAL_JACK_ENABLED
 ifndef LOCAL_JACK_PROGUARD_FLAGS
     LOCAL_JACK_PROGUARD_FLAGS := $(LOCAL_PROGUARD_FLAGS)
 endif
 LOCAL_JACK_PROGUARD_FLAGS := $(addprefix -include ,$(proguard_options_file)) $(LOCAL_JACK_PROGUARD_FLAGS)
-endif # LOCAL_USE_JACK
+endif # LOCAL_JACK_ENABLED
 
 endif  # LOCAL_RESOURCE_DIR
 
@@ -123,10 +123,10 @@
 	$(hide) find $(PRIVATE_SOURCE_INTERMEDIATES_DIR) -name R.java | xargs cat > $@
 
 $(LOCAL_BUILT_MODULE): $(R_file_stamp)
-ifeq ($(LOCAL_USE_JACK),true)
+ifdef LOCAL_JACK_ENABLED
 $(noshrob_classes_jack): $(R_file_stamp)
 $(full_classes_jack): $(R_file_stamp)
-endif # LOCAL_USE_JACK
+endif # LOCAL_JACK_ENABLED
 $(full_classes_compiled_jar): $(R_file_stamp)
 
 # Rule to build AAR, archive including classes.jar, resource, etc.