Extracts UserRecord out of UserSwitcherController.

1. Moves UserRecord to its own class
2. Makes UserRecord a data class and makes it completely immutable

Point (2) required the addition of some state in UserSwitcherController.

Bug: 243844359
Test: manually verified that the user switcher still works
Change-Id: Ic92c7ded00e250212bc40b83b03aaa1db314aecc