Android JNI generation: Set JNI namespace in build files

This CL removes the use of the @JNINamespace annotation and instead
sets the correct JNI namespace in the build file.

Bug: webrtc:8278
Change-Id: Ia4490399e45a97d56b02c260fd80df4edfa092bf
Reviewed-on: https://webrtc-review.googlesource.com/76440
Commit-Queue: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23299}

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

@@ -21,7 +21,6 @@ import javax.annotation.Nullable;
  * JS APIs: http://dev.w3.org/2011/webrtc/editor/webrtc.html and
  * http://www.w3.org/TR/mediacapture-streams/
  */
-@JNINamespace("webrtc::jni")
 public class PeerConnection {
   /** Tracks PeerConnectionInterface::IceGatheringState */
   public enum IceGatheringState {