Fix MediaPreparerTest bug

Untag MediaPreparer unit testing as a CTS_v2 artifact, and add
MediaPreparerTest to known failures file so that tests are not run
during CTS_v1.

bug:25850508
Change-Id: I2631e5828926efa6a292ef9f2856bccf97947afd
diff --git a/tests/expectations/knownfailures.txt b/tests/expectations/knownfailures.txt
index 5328f78..45fa081 100644
--- a/tests/expectations/knownfailures.txt
+++ b/tests/expectations/knownfailures.txt
@@ -349,5 +349,12 @@
     "android.view.cts.MotionEventTest#testReadFromParcelWithInvalidSampleSize"
   ],
   bug: 25652250
+},
+{
+  description: "unit testing for MediaPreparer lives within mediastress module",
+  names: [
+    "android.mediastress.cts.preconditions.MediaPreparerTest"
+  ],
+  bug: 25850508
 }
 ]