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}