Lower then bitrate so that the delta between the highest layer of the lower stream and lowest layer of the higher stream is not too large. Also fix a bug in vie_encoder where the codec was not perioducally updated unless a new bitrate estimate was triggered. BUG=webrtc:4172 Review-Url: https://codereview.webrtc.org/2791273002 Cr-Commit-Position: refs/heads/master@{#18232}