This CL wires everything up and enables actual setting of the max bitrate encoding parameter
on the video RTP sender.
The following changes were made
* Add maxbitrate property to the settings model and settings store. Make sure to store and
read the maxbitrate from storage (to persist between app launches and make testing easier)
* Fix setup of encoding parameters for the rtp sender as previous timing was not right.
* Fix header of RTCRtpSender to expose needed parameter
BUG=webrtc:6654
Review-Url: https://codereview.webrtc.org/2492693003
Cr-Commit-Position: refs/heads/master@{#15120}