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/MediaStreamTrack.java

@@ -13,7 +13,6 @@ package org.webrtc;
 import javax.annotation.Nullable;
 
 /** Java wrapper for a C++ MediaStreamTrackInterface. */
-@JNINamespace("webrtc::jni")
 public class MediaStreamTrack {
   public static final String AUDIO_TRACK_KIND = "audio";
   public static final String VIDEO_TRACK_KIND = "video";