Enable jack multithreading

Jack multithreading offer significant compile time improvement of
single project compilation and the problems it causes can be
avoided using "-j -l XX" instead of "-j YY" when running make.

Change-Id: I323f3d287467df1b598ceab6521cc71b329e0008
diff --git a/core/config.mk b/core/config.mk
index 61a198a..ac4f81e 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -427,7 +427,7 @@
 ifneq ($(ANDROID_JACK_EXTRA_ARGS),)
 DEFAULT_JACK_EXTRA_ARGS := $(ANDROID_JACK_EXTRA_ARGS)
 else
-DEFAULT_JACK_EXTRA_ARGS := -D sched.runner=single-threaded --sanity-checks off
+DEFAULT_JACK_EXTRA_ARGS := --sanity-checks off
 endif
 JILL := java -Xmx3500m -cp $(JILL_JAR) com.android.jill.Main
 PROGUARD := external/proguard/bin/proguard.sh