Require delegated cert installer and app restriction manager to exist

Enforce that apps with delegated powers to exist on device before
empowering them. This is consistent with DevicePolicyManagerService's
internal logic to clear the delegation power once the package is removed.
For delegated cert installer, only enforce this new restriction on
device admins targeting N or later.

Bug: 26233778
Change-Id: Ia8f45dfd5290958cebb36991c4b6baa03e8c28ae