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/VideoSource.java

@@ -15,7 +15,6 @@ import javax.annotation.Nullable;
 /**
  * Java wrapper of native AndroidVideoTrackSource.
  */
-@JNINamespace("webrtc::jni")
 public class VideoSource extends MediaSource {
   private final NativeCapturerObserver capturerObserver;