This CL makes the WebRTC Java Wrapper more modular and allows the android
users to build WebRTC without audio and video(DataChannel only).
The BUILD file in sdk/android/ is modified to support modular WebRTC.
The peerconnection_jni.cc is split into peerconnection_jni.cc, video_jni.cc,
video_renderer_jni.cc and ownedfactoryandthreads.h/cc.
Add new modular build targets to JNI layer: audio_jni, video_jni,
null_audio_jni, null_video_jni. The users can link with different
targets to for different WebRTC functionalities.
This is split from CL: https://codereview.webrtc.org/2854123003/TBR=magjed@webrtc.org
BUG=webrtc:7613
Review-Url: https://codereview.webrtc.org/2939203002
Cr-Commit-Position: refs/heads/master@{#18647}
af6293517f