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/Metrics.java

@@ -28,7 +28,6 @@ import java.util.Map;
 // Most histograms are not updated frequently (e.g. most video metrics are an
 // average over the call and recorded when a stream is removed).
 // The metrics can for example be retrieved when a peer connection is closed.
-@JNINamespace("webrtc::jni")
 public class Metrics {
   private static final String TAG = "Metrics";