PublicTransit: Set a Facility's Station when it's being entered.

This simplifies creation by not forcing a station to be created before
the Facility object.

Also fixes up & adds checks to ensure all facilities are properly
tracked in a Station.

Bug: None
Change-Id: If0c3e1acb7210922612ac701f2f4ba36a2f0a422
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5725519
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1330402}