[SB] Set up is<ChildView>Visible flows in CollapsedStatusBarViewModel.

We want to pull out all the business logic from
CollapsedStatusBarFragment and put it in interactors and view models
instead. This CL sets up three new flows in CollapsedStatusBarViewModel
and populates them based on disable flags:
 - isClockVisible
 - areNotificationIconsVisible
 - isSystemInfoVisible

The visibilities of those child views is dependent on a *lot* more than
just the disable flags, but this CL adds the scaffolding for filling in
those visibilities. Future CLs should *just* touch
CollapsedStatusBarInteractor and/or CollapsedStatusBarViewModel to add
the visibility nuances.

Bug: 364360986
Flag: com.android.systemui.status_bar_simple_fragment
Test: atest CollapsedStatusBarInteractorTest
CollapsedStatusBarViewModelImplTest

Change-Id: I1272d78d4435404272049ddcf7de89bd0290cae0