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
Show an empty view when there are no tasks on the desktop.
Bug: 267326725
Test: have a fullscreen task, open overview, observe empty view for
desktop tile
Test: have a desktop task, open overview, observe desktop task thumbnail
is shown in overview and empty view is hidden
Change-Id: Ifb6d96d42bf602e9cc9e397efc0e07e2accdf16b
