Prevent re-entrant calls to onPause

This can occur when launching an occluding activity where the user has
user the password bouncer. The IME animation begins, but is then
canceled in favor of the activity being launched, which begins a chain
of events that start a second call to pause.

Fixes: 236348317
Test: manual (follow steps in bug to launch occluding activity)
Change-Id: I158504872d41c96f1f6f27cab481945cd11c27e0