Gracefully handle slowly appearing Home apps.

When starting a Guest user for the first time, it can take quite
awhile before the Home app enables itself.  So if we're unlocked
and no other Home app is around, chill for 500ms and see if someone
showed up.

Bug: 26267450
Change-Id: I3c4e01ae3681c650ea90b20ffc21835ae264b5e9