Refactoring for better extendability

- Exposing members of PhoneStatusBar, StatusBarKeyguardViewManager and
  KeyguardBouncer to sub class;
- Add a configuable SystemUIFactory as class factory for components;
- Add logoutCurrentUser and switchToByUserId to UserSwitcherController;

BUG:22407003
Change-Id: I3902baf3c721d89217b27a6310c4202a198cb209