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/clear_vars.mk b/core/clear_vars.mk
index 614a7ca..da63891 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -62,6 +62,7 @@
 LOCAL_DROIDDOC_STUB_OUT_DIR:=
 LOCAL_DROIDDOC_TEMPLATE_DIR:=
 LOCAL_DROIDDOC_USE_STANDARD_DOCLET:=
+LOCAL_DX_FLAGS:=
 LOCAL_EMMA_COVERAGE_FILTER:=
 LOCAL_EMMA_INSTRUMENT:=
 LOCAL_EXPORT_C_INCLUDE_DEPS:=
@@ -94,8 +95,7 @@
 LOCAL_JACK_CLASSPATH:=
 LOCAL_JACK_COVERAGE_EXCLUDE_FILTER:=
 LOCAL_JACK_COVERAGE_INCLUDE_FILTER:=
-# full or incremental
-LOCAL_JACK_ENABLED:=full
+LOCAL_JACK_ENABLED:=$(DEFAULT_JACK_ENABLED) # '' (ie disabled), disabled, full, incremental
 LOCAL_JACK_FLAGS:=
 LOCAL_JACK_PLUGIN:=
 LOCAL_JACK_PLUGIN_PATH:=