[flexiglass] Hooks up real app state and logic in keyguard (1)

- Source auth method from KeyguardSecurityModel, changing it from flow to getter

Bug: 280883900
Test: Unit tests still pass.
Test: manually verified that changing the screen lock setting in
Settings, between PIN, password, and pattern, properly affects
Flexiglass in System UI such that the right version of the bouncer UI is
displayed.

Change-Id: Id457458806dc6ac2720abe21a77d5e50b83836fc