Make KeyguardStatusBarViewModel HydratedActivatable

Follow up to ag/33983794
Apply recommended architecture to avoid trashcan concurrency
Activate the model in ViewBinder
Use hydratedStateOf instead of StateFlow for isSignOutButtonVisible

Test: atest KeyguardStatusBarViewModelTest
Test: atest UserLogoutInteractorTest
Test: manual - Sign out button visible on Keyguard's status bar
Flag: com.android.systemui.sign_out_button_on_keyguard_status_bar
Bug: 392052370
Change-Id: I4e666b514a6a3d7248286a2684671ea109ee7481