PIN/PUK Views not showing after boot

An ill-timed onThemeChanged() event can occur after boot, which forces
the bouncer to be destroyed. In most cases this is fine, but after
boot this leaves the keyguard with nothing to show, as PIN/PUK views
are required to unlocked the device.

Bug: 182130809
Test: manual, reboot the device with SIM inserted
Change-Id: If7b86df15de29b295d86eff610da476b4dbee566