Android VideoRendererGui: Add dispose function

There is currently no way to dispose VideoRendererGui or VideoRendererGui.YuvImageRenderer. This CL adds functions to do so.

BUG=webrtc:4892

Review URL: https://codereview.webrtc.org/1273803002

Cr-Commit-Position: refs/heads/master@{#9710}

webrtc/examples/androidapp/src/org/appspot/apprtc/CallActivity.java

@@ -279,6 +279,7 @@ public class CallActivity extends Activity
       logToast.cancel();
     }
     activityRunning = false;
+    VideoRendererGui.dispose();
   }
 
   // CallFragment.OnCallEvents interface implementation.