Disable multiuser toggle when more than 1 user is created

It makes more sense to have the toggle as on permanently as long as there are other users on the device. Otherwise if we toggle off "Allow multiple users", other users get hidden. Also with current behaviour the any admin can switch the toggle and this can block any other user, including main, from being able to use the device.

Bug: 295183792
Test: atest UserManagerTest
Change-Id: Ibf926223deec9c231917e279017222ee2cfba14f