Add Sign out button on keyguard's status bar

Add Sign out button to keyguard's status bar that logs out
to system user.

The button is enabled by a config value and visible if log out
to system user action is available.

Test: manual - button visible on lockscreen with flexiglass on
Test: atest KeyguardStatusBarViewModelTest
Flag: com.android.systemui.sign_out_button_on_keyguard_status_bar
Bug: 392052370

Change-Id: I6e1442a54112dcaa343ec7279ac9df0a2dba3bc7