[Bouncer] fix user switch race condition

When switching from user with auth i.e. password to a user that has
swipe to unlock can experience a race condition where bouncer is not
dismissed when switching users from bouncer.  This happens when
switching users very quickly. This can be resolved by changing our
subscription to when a user changes.

Bug: 228901381
Test: Unit test and manual test
Change-Id: I7df7af986094f4719d39ee5d63ceafea8a812493