Make FooterViewModel children hold flows.

Instead of the parent view holding a flow of the child view, it makes
more sense for the child view to hold a flow of the visibility instead.
That way we're not triggering updates unnecessarily.

The footer view already knows to ignore duplicate updates for fields,
but if we can avoid triggering a downstream listener that's a small win
for performance. Plus it avoids holding constant values in a flow
unnecessarily.

Bug: 293167744
Test: manual + FooterViewModelTest
Flag: ACONFIG com.systemui.notifications_footer_refactor_flag DEVELOPMENT

Change-Id: Ic512b2fbdeb1d99c4aeeab1500221ae17d3e1087