Set videoCapturerStopped when PeerConnectionClient stopped

BUG=webrtc:6682

Review-Url: https://codereview.webrtc.org/2489213002
Cr-Commit-Position: refs/heads/master@{#15020}

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

@@ -540,6 +540,7 @@ public class PeerConnectionClient {
       } catch (InterruptedException e) {
         throw new RuntimeException(e);
       }
+      videoCapturerStopped = true;
       videoCapturer.dispose();
       videoCapturer = null;
     }