Delay color picker back navigation until apply is complete

Suspend the apply function until apply is complete and color
configuration is updated to delay proceeding actions, including back
navigation. Prevent double apply in the process by checking if an apply
is already in progress.

There is currently no loading indicator, and corner cases
where config change update does not happen is not currently handled.

Flag: com.android.systemui.shared.new_customization_picker_ui
Test: manually verified & unit tests
Bug: 350718581
Change-Id: Ie6ea517dfb634b6a801f077e5ff655da4e05f663