Use a mojo::StrongBindingSet to manage PermissionServiceImpls

      mojo::StrongBindingSet replaces the pattern of manually managing a list
of Mojo service implementations and registering connection error
handlers to remove entries from the list when their pipes are closed.

BUG=None

Review-Url: https://codereview.chromium.org/2842013002
Cr-Commit-Position: refs/heads/master@{#470992}