Properly hide footer buttons when label visible

Previously, this was handled in FooterView#setFooterLabelVisible
directly. Pre-footer refactor, every time the footer was updated
(including button visibility), setFooterLabelVisible would be called at
the end and essentially override button visibilities if needed. In the
new code, button visibility would be updated independently, which could
lead to buttons showing up over the label.

Instead, we should model this in the FooterViewModel and make sure the
buttons are never visible when the label is visible.

Fix: 332438711
Test: FooterViewModelTest + manual check that clear all button doesn't
appear on tangor over "unlock to see notifications" text
Flag: ACONFIG com.android.systemui.notifications_footer_view_refactor TEAMFOOD

Change-Id: I78c2db21b3d1b7dd0301065f61b85186c214d4d8