Hold a reference to AndroidVideoTrackSource while calling onFrameCaptured.

This makes it safe to deliver frames to the sink from VideoProcessor
even after setSink has been called with null reference without danger
of use after free.

Bug: b/148063550
Change-Id: Ib78f75ac49fc6117f744c55da1a4e671bbdcdf22
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/168160
Reviewed-by: Paulina Hensman <phensman@webrtc.org>
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30455}

sdk/android/api/org/webrtc/VideoProcessor.java

@@ -54,7 +54,7 @@ public interface VideoProcessor extends CapturerObserver {
 
   /**
    * Set the sink that receives the output from this processor. Null can be passed in to unregister
-   * a sink. After this call returns, no frames should be delivered to an unregistered sink.
+   * a sink.
    */
   void setSink(@Nullable VideoSink sink);