Clean .art files as well as .oat files.

The failure if you have a stale boot.art lying around and don't specify
-Ximage is deeply impenetrable.

Change-Id: I32ae1efccdcc5ab298384ecb764e64657679a37b
diff --git a/Android.mk b/Android.mk
index 809ee85..6beaadd 100644
--- a/Android.mk
+++ b/Android.mk
@@ -361,9 +361,13 @@
 .PHONY: clean-oat
 clean-oat:
 	rm -f $(ART_TEST_OUT)/*.oat
+	rm -f $(ART_TEST_OUT)/*.art
 	rm -f $(ART_CACHE_OUT)/*.oat
+	rm -f $(ART_CACHE_OUT)/*.art
 	adb shell rm $(ART_TEST_DIR)/*.oat
+	adb shell rm $(ART_TEST_DIR)/*.art
 	adb shell rm $(ART_CACHE_DIR)/*.oat
+	adb shell rm $(ART_CACHE_DIR)/*.art
 
 ########################################################################
 # cpplint target