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: EXEMPT bugfix
Test: atest DreamServiceTests
Change-Id: I286010553c87877cd5e50a19a295b4c54e928ebb