Fix issue #5320747: Disabling an app that is an active device administrator...

...can lead to an unsecure device

Now you can't disable device admins.

Or clear their data.

Or force stop them.

Or uninstall them (though the package manager also prevents uninstalling, might
as well disable this button along with everything else).

Change-Id: I69eefc2342e6f932908e9f5b87d3a601752810a4