Android: Generate JNI code for jni_helper, RtpSender, and VideoEncoderFactory

Bug: webrtc:8278
Change-Id: I6111e562cc5a8fe6c595fcb8dfd5202d46a08806
Reviewed-on: https://webrtc-review.googlesource.com/28622
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21140}

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

@@ -13,11 +13,11 @@ package org.webrtc;
 /** Factory for creating VideoEncoders. */
 public interface VideoEncoderFactory {
   /** Creates an encoder for the given video codec. */
-  public VideoEncoder createEncoder(VideoCodecInfo info);
+  @CalledByNative VideoEncoder createEncoder(VideoCodecInfo info);
 
   /**
    * Enumerates the list of supported video codecs. This method will only be called once and the
    * result will be cached.
    */
-  public VideoCodecInfo[] getSupportedCodecs();
+  @CalledByNative VideoCodecInfo[] getSupportedCodecs();
 }