Update MainSwitch on User Settings page

Turn the MainSwitchBar into MainSwitchPreference so that it is displayed as part of the page instead of floating above it.

Bug: 406107021
Test: atest MultiUserMainSwitchPreference
Flag: android.multiuser.change_switch_bar_into_main_preference
Change-Id: I4aac62624b456c9ce2acd757af4ddfc1e914eaf8