Improve protection against malicious admins.

- 2 second timeout until we give up on displaying a message,
  keeping us within the 5 second activity launch block time.
- Restart the activity launch block when we show the dialog.
- Use app ops to prevent the app of the currently displayed
  admin from putting windows on top while the settings screen
  is running.

Change-Id: I631e8896ec7539c2de3ff36f183e67e36bcc37ca