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/RtpTransceiver.java

@@ -31,7 +31,6 @@ import org.webrtc.RtpParameters.Encoding;
  * <p>WebRTC specification for RTCRtpTransceiver, the JavaScript analog:
  * https://w3c.github.io/webrtc-pc/#dom-rtcrtptransceiver
  */
-@JNINamespace("webrtc::jni")
 public class RtpTransceiver {
   /** Java version of webrtc::RtpTransceiverDirection - the ordering must be kept in sync. */
   public enum RtpTransceiverDirection {