Use PRODUCT_OUT instead of ANDROID_PRODUCT_OUT

This fixes an error when running art/tools/buildbot-build.sh:
art/Android.mk:137: error: ANDROID_PRODUCT_OUT is obsolete. Use
PRODUCT_OUT instead.

Test: successfully executed buildbot-build.sh
Change-Id: If269bb7b5b53c2940b4299c953f8fd9c27dc2c50
diff --git a/Android.mk b/Android.mk
index cb0b709..0a90a0b 100644
--- a/Android.mk
+++ b/Android.mk
@@ -135,11 +135,11 @@
 else
 test-art-target-sync: $(TEST_ART_TARGET_SYNC_DEPS)
 	$(TEST_ART_ADB_ROOT_AND_REMOUNT)
-	adb wait-for-device push $(ANDROID_PRODUCT_OUT)/system $(ART_TEST_ANDROID_ROOT)
+	adb wait-for-device push $(PRODUCT_OUT)/system $(ART_TEST_ANDROID_ROOT)
 # Push the contents of the `data` dir into `/data` on the device.  If
 # `/data` already exists on the device, it is not overwritten, but its
 # contents are updated.
-	adb push $(ANDROID_PRODUCT_OUT)/data /
+	adb push $(PRODUCT_OUT)/data /
 endif
 endif