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_unittests/audio_device/audio_device_unittest.cc

@@ -50,7 +50,7 @@ using ::testing::Return;
 
 namespace webrtc {
 
-namespace android_adm {
+namespace jni {
 
 // Number of callbacks (input or output) the tests waits for before we set
 // an event indicating that the test was OK.
@@ -1135,6 +1135,6 @@ TEST_F(AudioDeviceTest, DISABLED_MeasureLoopbackLatency) {
   latency_audio_stream->PrintResults();
 }
 
-}  // namespace android_adm
+}  // namespace jni
 
 }  // namespace webrtc