Re-enable dx support

This is a partial revert of commits:
858657366fa6 Remove support of disabling Jack.
3ae78612522e Remove javac support in host dex rules.
22313f2b2a4a Remove rules for building dex with dx

Test: builds
Change-Id: Ie12d743cbe978bdeb030910848b67f5945a4fec8
diff --git a/core/static_java_library.mk b/core/static_java_library.mk
index 258c5f8..eeef4ce 100644
--- a/core/static_java_library.mk
+++ b/core/static_java_library.mk
@@ -75,10 +75,12 @@
 
 LOCAL_PROGUARD_FLAGS := $(addprefix -include ,$(proguard_options_file)) $(LOCAL_PROGUARD_FLAGS)
 
+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_JACK_ENABLED
 
 R_file_stamp := $(intermediates.COMMON)/src/R.stamp
 LOCAL_INTERMEDIATE_TARGETS += $(R_file_stamp)
@@ -164,10 +166,13 @@
 	$(hide) find $(PRIVATE_SOURCE_INTERMEDIATES_DIR) -name R.java | xargs cat > $@
 endif  # LOCAL_USE_AAPT2
 
-$(LOCAL_BUILT_MODULE) \
-$(full_classes_compiled_jar) \
-$(noshrob_classes_jack) $(full_classes_jack) $(jack_check_timestamp) \
-  : $(R_file_stamp)
+$(LOCAL_BUILT_MODULE): $(R_file_stamp)
+ifdef LOCAL_JACK_ENABLED
+$(noshrob_classes_jack): $(R_file_stamp)
+$(full_classes_jack): $(R_file_stamp)
+$(jack_check_timestamp): $(R_file_stamp)
+endif # LOCAL_JACK_ENABLED
+$(full_classes_compiled_jar): $(R_file_stamp)
 
 # Rule to build AAR, archive including classes.jar, resource, etc.
 built_aar := $(intermediates.COMMON)/javalib.aar