Prevent wrong scalability mode from being used when base layer inactive.

In SimulcastEncoderAdapter, use the "global" scalabilty mode from the
VideoCodec config struct for a stream only if there is single active
simulcast layer AND that layer is the first layer in the list.

Bug: webrtc:510393737
Change-Id: I37cfd4861b0b8898db3d0f5e13ec5b1f8d5cc1c1
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/470520
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47648}