[RFC8888] Enable ALR probing when Congestion Control Feedback is enabled.

When the RFC8888 Congestion Control Feedback (CCFB) experiment is
enabled, send-side bandwidth estimation (BWE) is active, but TWCC might
not be configured (as feedback is sent via CCFB instead).

This CL updates VideoSendStreamImpl to check if CCFB is enabled when
deciding whether to apply ALR probing and pacing settings. If CCFB is
enabled, we allow ALR probing and queue time limits to be configured,
but we do NOT set the pacing factor on the transport controller (as
pacing is handled exclusively by the congestion controller in this
mode).

This CL also refactors the tests to remove the use of the internal
configured_pacing_factor_ member, allowing it to be removed from
VideoSendStreamImpl.

Bug: webrtc:522844909, chromium:523643492
Change-Id: I8920e0551b5385de5426a37c926080d90883ce87
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/481601
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48000}