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/PeerConnectionFactory.java

@@ -20,7 +20,6 @@ import org.webrtc.audio.LegacyAudioDeviceModule;
  * Java wrapper for a C++ PeerConnectionFactoryInterface.  Main entry point to
  * the PeerConnection API for clients.
  */
-@JNINamespace("webrtc::jni")
 public class PeerConnectionFactory {
   public static final String TRIAL_ENABLED = "Enabled";
   @Deprecated public static final String VIDEO_FRAME_EMIT_TRIAL = "VideoFrameEmit";