lacros: Refactor NotificationPlatformBridgeChromeOs

Lacros needs to support two notification pathways:
* In-process, for the ash-chrome binary
* Out-of-process, for the lacros-chrome binary

However, both of these pathways have some shared behavior around
handling multiple profiles (for the pre-lacros ash case) and transient
notifications. This code lives in NotificationPlatformBridgeChromeOs
and we want to reuse it.

1. Introduce MessageCenterClientLacros. This is a stub for now. In
   a follow-up CL it will send notifications over mojo from lacros
   to ash.
2. Change NotificationPlatformBridgeChromeOs so it supports different
   |impl_| helpers for ash and lacros.
3. Refactor ChromeAshMessageCenterClient to inherit from
   NotificationPlatformBridge. This allows NPBChromeOs to have a
   generic |impl_| for both helpers, and avoids introducing a new
   abstract interface class.

See go/lacros-notifications for overall plan.

Bug: 1113889
Test: existing unit_tests and browser_tests
Change-Id: I159f410f3d84cc6e9bde0b3757015691f2289419
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343217
Reviewed-by: Jun Mukai <mukai@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796096}