Annotate AppRTCMobile with @Nulllable.

Bug: webrtc:8881
Change-Id: I32390b1e2586415757f0453b60d35b23160d2862
Reviewed-on: https://webrtc-review.googlesource.com/63641
Reviewed-by: Paulina Hensman <phensman@webrtc.org>
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22580}

examples/androidapp/src/org/appspot/apprtc/RtcEventLog.java

@@ -71,4 +71,4 @@ public class RtcEventLog {
     state = RtcEventLogState.STOPPED;
     Log.d(TAG, "RtcEventLog stopped.");
   }
-}
+}