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.cc b/webrtc/examples/unityplugin/classreferenceholder.cc
index 8814699..b009ce4 100644
--- a/webrtc/examples/unityplugin/classreferenceholder.cc
+++ b/webrtc/examples/unityplugin/classreferenceholder.cc
@@ -36,12 +36,12 @@
 
 void LoadGlobalClassReferenceHolder() {
   RTC_CHECK(g_class_reference_holder == nullptr);
-  g_class_reference_holder = new ClassReferenceHolder(webrtc_jni::GetEnv());
+  g_class_reference_holder = new ClassReferenceHolder(webrtc::jni::GetEnv());
 }
 
 void FreeGlobalClassReferenceHolder() {
   g_class_reference_holder->FreeReferences(
-      webrtc_jni::AttachCurrentThreadIfNeeded());
+      webrtc::jni::AttachCurrentThreadIfNeeded());
   delete g_class_reference_holder;
   g_class_reference_holder = nullptr;
 }