VideoCapturerAndroid: Release queued camera frames when stopCapture() is called

BUG=webrtc:4892

Review URL: https://codereview.webrtc.org/1285823002

Cr-Commit-Position: refs/heads/master@{#9709}

talk/app/webrtc/java/src/org/webrtc/VideoCapturerAndroid.java

@@ -898,6 +898,8 @@ public class VideoCapturerAndroid extends VideoCapturer implements PreviewCallba
 
     void stopReturnBuffersToCamera() {
       this.camera = null;
+      queuedBuffers.clear();
+      // Frames in |pendingBuffers| need to be kept alive until they are returned.
       Log.d(TAG, "stopReturnBuffersToCamera called."
             + (pendingBuffers.isEmpty() ?
                    " All buffers have been returned."