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/config.mk b/core/config.mk
index 2f43f46..f40f697 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -605,6 +605,11 @@
 FAT16COPY := build/tools/fat16copy.py
 CHECK_LINK_TYPE := build/tools/check_link_type.py
 
+ifeq ($(ANDROID_COMPILE_WITH_JACK),true)
+DEFAULT_JACK_ENABLED:=full
+else
+DEFAULT_JACK_ENABLED:=
+endif
 ifneq ($(ANDROID_JACK_EXTRA_ARGS),)
 JACK_DEFAULT_ARGS :=
 DEFAULT_JACK_EXTRA_ARGS := $(ANDROID_JACK_EXTRA_ARGS)