Redesign user switcher

- Change user, guest and add user icons.
- Add light and dark theme support for both quicksettings and keyguard
user switcher.

Bug: 150286182
Test: Check both lock screen user switcher and quick settings switcher manually on tablet and phone (phone doesn't have a lock screen user switcher).
Change-Id: I18cd12e85ab236f3d541998673c375393b4ad3f4