Add reason for showing the bouncer

It can be confusing what is actually triggering the bouncer
to show. The code to show the bouncer has been incorrect
at times, leading to bad transitions. Add better logging.

Bug: 395016382
Test: atest SystemUITests
Flag: EXEMPT bugfix
Change-Id: I800815c289b6e423883a3a1b5417af1ea63b08ff