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/MediaCodecVideoEncoder.java

@@ -37,7 +37,6 @@ import org.webrtc.VideoFrame;
 // This class is an implementation detail of the Java PeerConnection API.
 @TargetApi(19)
 @SuppressWarnings("deprecation")
-@JNINamespace("webrtc::jni")
 public class MediaCodecVideoEncoder {
   // This class is constructed, operated, and destroyed by its C++ incarnation,
   // so the class and its methods have non-public visibility.  The API this