Changes in VideoCapturerAndroid.

- Do not handle more than one camera switch request at a time
to avoid blocking camera thread with multiple switch requests.
- Add a callback to notify when camera switch has been done.

R=perkj@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46859004

Cr-Commit-Position: refs/heads/master@{#8978}

talk/examples/android/src/org/appspot/apprtc/PeerConnectionClient.java

@@ -818,7 +818,7 @@ public class PeerConnectionClient {
       return;  // No video is sent or only one camera is available or error happened.
     }
     Log.d(TAG, "Switch camera");
-    videoCapturer.switchCamera();
+    videoCapturer.switchCamera(null);
   }
 
   public void switchCamera() {