Change Fullscreen UserSwitcher into a dialog

This is done to retain the last-active-app-resumption when switching users.

Also mitigates some issues with the previous activity behavior in "lockTaskMode".

Bug: b/270495254
Fixes: b/274122173
Test: UserInteractorTest.kt
Test: manual - switch user from chip bar to other non-supervised user
Test: manual - switch user to locked-down supervised user. verify that user switcher still appears on locked-down child user and that the user can switch back to the main user.
Test: manual - switch user to guest user, and back
Test: manual - open fullscreen user switcher, turn off screen. verify that user switcher is dismissed.
Test: manual - verify user switcher fullscreen is the expected UI in portrait and landscape.
Change-Id: Icc65e464c4f08817fc3668b2e898dadfc7cea4aa