Lacros: Simplify handling of system URLs

- Get rid of the custom os:// URL parsing/santization code. Thanks to
  changes a while ago, we never need to sanitize an os:// URL while
  keeping it an os:// URL. Hence we can just convert it to a chrome://
  URL first and then sanitize that.
- Also split URL sanitization from Lacros-to-Ash conversion for clarity
  and avoiding duplicate work. Do sanitiziation only where it matters,
  namely in UrlHandlerAsh.
- Make Ash's CanHandleUrl and UrlHandlerAsh expect an Ash URL. Do the
  conversion at call sites if needed.
- Simplify OsUrlHandlerSystemWebAppDelegate::IsUrlInSystemAppScope
  based on the above.
- Rename some functions for clarity and consistency.
- Remove/update some outdated documentation.
- Remove an obsolete code path from lacros_url_handling's
  IsUrlAcceptedByAsh. It was needed for older Ash, which are now
  outside the supported version skew range.

This is in preparation for addressing the referenced bug.

Bug: chromium:1445145
Change-Id: I3eb734859fae264587d8e9241963a4de59d33888
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4914773
Reviewed-by: Orin Jaworski <orinj@chromium.org>
Reviewed-by: Stefan Kuhne <skuhne@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1214624}