Android: Generate JNI code for VideoSink and VideoEncoder

This is the first CL to start generating JNI code. It has updated two of
the most recent classes to use JNI code generation.

Bug: webrtc:8278
Change-Id: I1b19ee78c273346ceeaa0401dbdf8696803f16c7
Reviewed-on: https://webrtc-review.googlesource.com/3820
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19994}

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

@@ -19,5 +19,5 @@ public interface VideoSink {
    * this function returns. Each call to retain() should be followed by a call to frame.release()
    * when the reference is no longer needed.
    */
-  void onFrame(VideoFrame frame);
+  @CalledByNative void onFrame(VideoFrame frame);
 }