Make RtpVideoSender's SSRC-to-module map const

ConfigureSsrcs() filled in the map after construction, so it couldn't be
const even though it never changes afterwards. Build it in the
initializer list instead. Use a flat_map, since the map is small, built
once and only used for lookups.

Bug: webrtc:42223727
Change-Id: Ib306aaca6cfd81415c7ced7aaf02c4af4539a9e3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/506160
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48757}