[sb] add areaTint function to home status bar view model

This function returns a flow, based on displayId, which emits a new tint
calculator every time the dark region of the screen changes. The emitted
value is simply a functional interface from Rect (view bounds) -> tint

Test: HomeStatusBarViewModelImplTest
Bug: 364360986
Flag: com.android.systemui.status_bar_root_modernization
Change-Id: I1087f07b995cfda42cf9503ff734f8b4b7856910