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/native_api/jni/class_loader.cc

@@ -32,7 +32,7 @@ namespace {
 class ClassLoader {
  public:
   explicit ClassLoader(JNIEnv* env)
-      : class_loader_(Java_WebRtcClassLoader_getClassLoader(env)) {
+      : class_loader_(jni::Java_WebRtcClassLoader_getClassLoader(env)) {
     class_loader_class_ = reinterpret_cast<jclass>(
         env->NewGlobalRef(env->FindClass("java/lang/ClassLoader")));
     CHECK_EXCEPTION(env);