Add tests for UsageStats API to knownfailures.

The tests are not yet ready to be added to the offical test plan

bug: 17536113

Change-Id: I6bafcb47db3d00f4865495c97d7d0f177bd5a518
diff --git a/tests/expectations/knownfailures.txt b/tests/expectations/knownfailures.txt
index c47d53b..dd8660f 100644
--- a/tests/expectations/knownfailures.txt
+++ b/tests/expectations/knownfailures.txt
@@ -1,5 +1,12 @@
 [
 {
+  description: "the UsageStats is not yet stable enough",
+  names: [
+    "android.app.usage.cts.UsageStatsTest"
+  ],
+  bug: 17536113
+},
+{
   description: "the ConnectivityConstraintTest are not yet stable",
   names: [
     "android.jobscheduler.cts.ConnectivityConstraintTest"