[dPWA] Support uninstallation of individual install URLs for externally managed web apps

There was no code path for removing an install URL from an externally
managed web app. Instead we were removing the associated install
source entirely. This CL fixes that behavior.

As part of this CL WebAppUninstallCommand is split up into three
different removal jobs:
- RemoveInstallUrlJob
- RemoveInstallSourceJob
- RemoveWebAppJob
Each job may invoke another one within the overall command's
AllAppsLock. These jobs basically make up an async function call
stack scoped within the lifetime of WebAppProvider.

https://chromium-review.googlesource.com/c/chromium/src/+/4541923
attempted to do the same thing but was reverted due to crashes in
Lacros. I was not able to repro these crashes and it is my hope that
this completely different approach doesn't have the same crash bug.

Bug: 1427340
Change-Id: I67a98969874ec0157286b81f4b6ef64e676e1a35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4564896
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Jon Mann <jonmann@chromium.org>
Reviewed-by: Glen Robertson <glenrob@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Roland Bock <rbock@google.com>
Cr-Commit-Position: refs/heads/main@{#1156645}