When pressing back from fingerprint enrollment flow, Settings will get
crashed. That was caused by calling a destroied fragment manager to
create an error dialog.
Bug: 184234884
Test: manual test
1) Settings -> Security -> Fingerprint
2) Following the enrollment flow, then pressing back from either find
sensor or enrolling page
3) Observe and see if there's no crash
Change-Id: I2028df0c6a65e8e960a627161834660681b7b70a
05c2a2bf41