Clean up logic around user icons

As part of CarUserManagerHelper refactoring effort, I am deleting APIs
around UserIcons.  As a result, I've also cleaned up some of the logic
around getting UserIcons for users.

Also updated logic to more closely match Settings logic with scaling and
rounded icons.

Bug: 131776619
Test: atest CarSettingsRoboTests
Test: Manually open both user pickers and play with UI

Change-Id: If251cf3662529b69b2a250c76ae1c715b2fd84e7