Set desktop icon to always use black for the icon

Desktop icon in overview was using a themed color for the icon. But the
background is always a white circle. When using a dark theme, the themed
primary text color is too light. Fixing the color of the icon to be
black always.

Bug: 267326722
Test: enable dark mode, open overview and check the desktop tile icon
Change-Id: Ibcfdfd92ffe2db41cf6ed75c3da8f6457773c2bf