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-Original-Commit-Position: refs/heads/master@{#19569}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a3d4f68fb49decd3a365e9a60c9d530c691d6b73
78 files changed