Introducing Clear-all button on Overview

It’s an initial implementation, i.e. no fancy effects.
It shares a parent (LauncherRecentsViewContainer) with RecentsView.
The button is centered in clear_all_button_container, which gets
positioned programmatically to the right of the last task. (RTL polish
will be a separate CL as well).

Bug: 72222505
Change-Id: Ia912908a93a30c2f51450ccf0f97c7495e7916d5
Test: Manual