Annotate libjingle_peerconnection_java with @Nullable.

Bug: webrtc:8881
Change-Id: Ida2ef6c003567d19529c21629c916ed40e8de3a6
Reviewed-on: https://webrtc-review.googlesource.com/63380
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Paulina Hensman <phensman@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22563}

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

@@ -11,6 +11,7 @@
 package org.webrtc;
 
 import android.media.MediaRecorder;
+import javax.annotation.Nullable;
 
 /**
  * Base interface for camera1 and camera2 implementations. Extends VideoCapturer with a