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}

examples/androidnativeapi/BUILD.gn

@@ -28,6 +28,7 @@ generate_jni("generated_jni") {
     "java/org/webrtc/examples/androidnativeapi/CallClient.java",
   ]
   jni_package = ""
+  namespace = "webrtc_examples"
   jni_generator_include = "//sdk/android/src/jni/jni_generator_helper.h"
 }