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/VideoFrame.java

@@ -14,6 +14,7 @@ import android.graphics.Matrix;
 import android.opengl.GLES11Ext;
 import android.opengl.GLES20;
 import java.nio.ByteBuffer;
+import javax.annotation.Nullable;
 
 /**
  * Java version of webrtc::VideoFrame and webrtc::VideoFrameBuffer. A difference from the C++