Support customizing icon color for externally items.

When a setting is injected to homepage, we now allow the setting
provider customize the background color for its icon.

If no custom bg color is provided, we fallback to existing behavior.

This behavior is only needed in homepage, because we don't have any
tinting logic in other pages so external setting providers can just
provide arbitrary icon.

Bug: 77188317
Test: robotests
Change-Id: I7d4b512fafc28537de46192026ce4bbe51b498c4