Update lock screen user switcher on settings update and re-create view stub...

Update lock screen user switcher on settings update and re-create view stub after disabling multi-user

NotificationPanelViewController inflates user switcher
view stub after enabling multi-user and removes user
switcher view after disabling this setting. After enabling
it again it crashed when trying to inflate view stub as
the view was removed. Fixed by re-inserting view stub
after disabling user switcher.

Another issue was that user switcher is not updated
immediately after changing the multi-user setting and after
switching displays. Fixed by adding settings observer and
listening for screen size changes in the controller.

Test: atest com.android.systemui.statusbar.phone.NotificationPanelViewTest
Fixes: 186728895
Change-Id: I1957abf952aff938406f4cabf4c83cd2977099e8