Don't crash when SDP describes fewer send layers

RtpSenderBase::SetSsrcTask() applies the encodings the application
configured via addTransceiver() to the media channel once a local
description has been applied. The media channel derives its encodings
from the negotiated StreamParams, so the number of layers it knows
about is whatever the local description describes.

When the description describes fewer layers than the application asked
for, for instance because simulcast was removed by SDP munging, or
because the sender is attached to an ssrc the media channel has no send
stream for, the two views disagree and the RTC_CHECK_GE in the worker
thread lambda terminates the process. The mismatch is a negotiation
outcome, not memory corruption, so crashing is too strong a response
and the SDP remains authoritative either way.

Apply the layers that the media channel knows about, log a warning for
the ones that are dropped, and leave the channel untouched when it has
no send stream for the ssrc, which is the same treatment ssrc 0 gets.

Bug: webrtc:558756689, webrtc:562462476
Change-Id: I1775221261eae116f55d5fc45e3e5a7417db88b0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/503820
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48615}