This cl move calculation of stats for prefered_media_bitrate_bps from webrtcvideoengine2.GetStats to SendStatisticsProxy::OnEncoderReconfigured. This aligns better with how other send stats are reported and is needed as a prerequisite for moving video encoder configuration due to video resolution change from WebRtcVideoEngine2 to ViEEncoder. BUG=webrtc:6371 R=mflodman@webrtc.org, sprang@webrtc.org Review URL: https://codereview.webrtc.org/2368223002 . Cr-Commit-Position: refs/heads/master@{#14431}