Simulcast screenshare adjustment to temporal layers, bitrate

Change experimental max bitrate setting from 1.6Mbps to 1.25Mbps in
order to allow a larger fraction of participants to receive this layer.

Add a new field trial to allow setting the number of temporal layers for
the high-quality simulcast stream in screensharing separately from the
temporal layer count for regular video.

Bug: webrtc:9477
Change-Id: I1341b774f870c50710901da24963bd3ede96ffd8
Reviewed-on: https://webrtc-review.googlesource.com/95101
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24356}