Android: Generate all C++ -> Java JNI code for VideoEncoder

The first example CL for generating JNI code
(https://webrtc-review.googlesource.com/c/src/+/4500) seems to stick, so
this CL updates the rest of the VideoEncoder. The JNI code for
Java -> C++ is still done manually.

This CL puts the necessary helper Java methods in a class called
VideoEncoderWrapper.

Bug: webrtc:8278
Change-Id: Ic3a6defe59c094f67ffd8ea86d6c272c676980ae
Reviewed-on: https://webrtc-review.googlesource.com/20871
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20587}

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

@@ -35,6 +35,7 @@ public enum VideoCodecStatus {
     this.number = number;
   }
 
+  @CalledByNative
   public int getNumber() {
     return number;
   }