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/java_common.mk b/core/java_common.mk
index dbdea26..8fc4fb6 100644
--- a/core/java_common.mk
+++ b/core/java_common.mk
@@ -320,6 +320,7 @@
 ###########################################################
 # JACK
 ###########################################################
+ifdef LOCAL_JACK_ENABLED
 ifdef need_compile_java
 
 LOCAL_JACK_FLAGS += -D jack.java.source.version=$(LOCAL_JAVA_LANGUAGE_VERSION)
@@ -373,6 +374,7 @@
 $(LOCAL_INTERMEDIATE_TARGETS): PRIVATE_JARJAR_RULES := $(LOCAL_JARJAR_RULES)
 
 endif  # need_compile_java
+endif # LOCAL_JACK_ENABLED
 
 
 ###########################################################