Move StateFlow to ViewModel scope

Follow up to ag/33983794.
After making KeyguardStatusBarViewModel activateable, move collection
of isVisible StateFlow to viewmodel's activation scope.

Test: atest KeyguardStatusBarViewModelTest
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: I8762cda73fe70ca1300584a144b04f1cf69c141c