Guard against a nonexistent dream settings activity.

We already guard against a dream specifying a settings activity in a
different package. This change guards against a settings activity that
doesn't exist (which currently causes a settings app crash when a user
taps the 'Customize' button.

Bug: 330298841
Flag: NA
Test: atest DreamServiceTests

Change-Id: I276cd19587b99db7c6aebf0ff5896cb4580bf157