Android: Replace webrtc_jni namespace with nested jni namespace

This CL replaces:
namespace webrtc_jni {
with:
namespace webrtc {
namespace jni {

The main benefit is that we don't have to use the webrtc:: qualifier
inside the jni namespace so we can reduce some clutter.

BUG=None

Review-Url: https://codereview.webrtc.org/3009613002
Cr-Commit-Position: refs/heads/master@{#19569}
diff --git a/webrtc/examples/unityplugin/classreferenceholder.h b/webrtc/examples/unityplugin/classreferenceholder.h
index a3f83e8..7d9a71b 100644
--- a/webrtc/examples/unityplugin/classreferenceholder.h
+++ b/webrtc/examples/unityplugin/classreferenceholder.h
@@ -8,7 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-// This is a supplement of webrtc_jni::ClassReferenceHolder.
+// This is a supplement of webrtc::jni::ClassReferenceHolder.
 // The purpose of this ClassReferenceHolder is to load the example
 // specific java class into JNI c++ side, so that our c++ code can
 // call those java functions.