[User Switcher] Make add button pillshaped.

Make add button in fullscreen userswitcher pillshaped by adding a 20dp
corner radius. Remove the style impl and add a new drawable to use.

Test: Opened fullscreen userswitcher and observed the menu in portrait
and landscape.
Bug: 256151842

Change-Id: I6af041fca42c7600a36a94b768009d4acef27fac