Update screen simulcast config and fix periodic encoder param update
Update screen simulcast config and fix periodic encoder param update

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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#18232}

ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc