Initialize data required by TrackMediaInfoMap on the signaling thread

Move the initialization of track mappings for `TrackMediaInfoMap` to the
constructor. Gathering the data for constructing the map is now done for
the most part on the signaling thread, in particular ensuring that
`RtpSenderBase::track()` and `ssrc()` are accessed in a thread-safe
manner.

Previously, the mappings were done during `Initialize()` on the worker
thread, which breaks thread-safety guarantees (track() should be
accessed on the signaling thread for example). The Initialize method is
now gone and all members of TrackMediaInfoMap have been made const.

Updates the `RTCStatsCollector` to gather `RtpSenderSignalInfo` and
`RtpReceiverSignalInfo` on the signaling thread and pass them to the
`TrackMediaInfoMap` constructor.

Also updates the `TrackMediaInfoMap` API to return `TrackId` and
`AttachmentId` via `GetTrackIdBySsrc` and `GetAttachmentIdBySsrc`,
replacing the previous methods that returned raw track pointers.

Bug: none
Change-Id: Ia15d74d064fcd6c4b906e3c6116e17194a54cd78
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/437020
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46550}