Libcore failures for buildbot should now be flagged device_testdex.

Test: run-libcore-tests
Change-Id: I7d2c027aef7e05556b5cae9c3d8dd428b2040b4f
diff --git a/tools/libcore_failures.txt b/tools/libcore_failures.txt
index 3f9ceea..a2777e8 100644
--- a/tools/libcore_failures.txt
+++ b/tools/libcore_failures.txt
@@ -227,7 +227,7 @@
 {
   description: "Apex related",
   result: EXEC_FAILED,
-  modes: [device],
+  modes: [device_testdex],
   bug: 122642227,
   names: [
     "libcore.java.lang.SystemTest#testSystemProperties_mutable",