Make Keyguard a library and make StatusBar the new Keyguard.

This change achieves a couple of things:
- Let Keyguard be a library, so we can use it in SystemUI.
- Introduce FLAG_KEYGUARD for windows and deprecate TYPE_KEYGUARD. Make
all the TYPE_KEYGUARD behaviour dependant on the flag.
- Implement a new KeyguardService in SystemUI, and bind that service
from PhoneWindowManager.
- Introduce BaseStatusBar.setKeyguardState and inflate
KeyguardSimpleHostView there and use FLAG_KEYGUARD for the window, such
that the status bar window essentially gets the Keyguard.

Bug: 13635952
Change-Id: I059d80d8b9b9818a778ab685f4672ea2694def63