[BMO] Support installing an app locally on chrome://apps

This change adds the installAppLocally support to chrome://apps. There
is no current way to set the 'locally_installed' attribute to true other
than through this 'ForTesting' method, but that method looks appropriate
for this use case.

This CL changes that method to not be ForTesting, and calls the
appropriate follow up steps using the WebApplication systems (creating
shortcuts and registering file handlers).

This is the final migration of AppLauncherHandler to use the
BMO WebAppProvider system.

R=calamity@chromium.org, loyso@chromium.org

Bug: 1009302
Change-Id: I0f2c3211a2d74f67c5de637f96129fae2398118d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2171845
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#765339}