Add support for custom user-switch UI

Given config_customUserSwitchUi, AM/UserController will not show any
UI during user-switch (no dialog or screen-freeze).

Provides a mechanism (WM.setSwitchingUser) by which a custom user-switch
UI can notify WM/Keyguard when it expects a user-switch operation to
be running.

Bug: 29329555
Change-Id: Ic903fc251d7ec3a54bc6a77906d3afa45a6a5fac