swa: Make SystemWebAppManager a KeyedService.

Introduce TestSystemWebAppManagerCreator.

`Profile` (BrowserContext) now owns SystemWebAppManager.

WebAppProvider doesn't depend on SystemWebAppManager anymore.

Bug: 1321984
Change-Id: I3f92006703ae4abe2a8ee69d4cd8d23294021714
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3685168


Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Jiewei Qian <qjw@chromium.org>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1012847}