Change taskbar corner roundness when entering overview

- Roundness will be 0 in Launcher, and 1 in an app

Bug: 250645563
Test: With and without taskbar in overview, switch between home/overview/app and observe the round corners above taskbar
Test: Ensure corner above taskbar is correct in overview and in app when chat bubble is active
Change-Id: I1f4911626c8e79fce8d81a76a9bd0d0670d111da