Redirect jack warnings to stdout

Bug: 19715777
Change-Id: If651a35f3579dd6e976eeb8a063b0eefdfaa992a
diff --git a/core/config.mk b/core/config.mk
index ac43d41..c01f0a6 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -441,7 +441,7 @@
 ifneq ($(ANDROID_JACK_EXTRA_ARGS),)
 DEFAULT_JACK_EXTRA_ARGS := $(ANDROID_JACK_EXTRA_ARGS)
 else
-DEFAULT_JACK_EXTRA_ARGS := --sanity-checks off
+DEFAULT_JACK_EXTRA_ARGS := --sanity-checks off -D jack.reporter.level.file=error=--,warning=-
 endif
 JILL := java -Xmx3500m -cp $(JILL_JAR) com.android.jill.Main
 PROGUARD := external/proguard/bin/proguard.sh