Show message center on lock screen
This patch does:
- Show the message center button on tray on the lock screen
- Show the message to encourage user to log in when user opens message center on lock screen
- Add SetLockedState(bool) and IsLockedState() method into MessageCenter class.
- Don't make notifications read when opening the message center on lock screen
- Add tests
And this patch also fixes the bug that this makes blocked notifications read wrongly.