DPWA: encapsulate sub managers in OsIntegrationManager.

Removes places that access AppShortcutsManager and FileHandlerManager
directly, limit the suraface of sub managers to only expose a subset
of getter through OsIntegrationManager.

Bug: 1098027
Change-Id: I1231727935b2c05f9083df0f7c9bff0f1ffa7a85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2350324


Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Phillis Tang <phillis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805893}