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