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/src/java/org/webrtc/NativeCapturerObserver.java

@@ -16,7 +16,6 @@ import javax.annotation.Nullable;
  * Implements VideoCapturer.CapturerObserver and feeds frames to
  * webrtc::jni::AndroidVideoTrackSource.
  */
-@JNINamespace("webrtc::jni")
 class NativeCapturerObserver implements VideoCapturer.CapturerObserver {
   // Pointer to webrtc::jni::AndroidVideoTrackSource.
   private final long nativeSource;