[Dual Shade] Separate composables for the toolbar buttons + refactoring.

This removes the background color for the buttons in Dual Shade, to
match the UX spec, and increases the size of the toolbar buttons in Dual
Shade to 24dp.

As a nice side effect, this simplifies the composables and view-models
of both the footer actions (non-Dual Shade) and the toolbar, and
consolidates color determination.

BONUS: Remove FooterActionsViewModel's and ToolbarViewModel's dependency
on the shade mode.

Fix: 415277314
Test: Manually tested by opening the QS shade and observing that the
 toolbar buttons are rendered without a background color.
Flag: com.android.systemui.scene_container
Change-Id: Iac68d38208689e0c3dae25503ff51b08cd6e3d33