Actually launch onboarding flow if needed

Launch OnboardingActivity for exported activities
(DialtactsActivity, CallDetailsActivity) to ensure that

a) We prompt the user to make us the default dialer at least once
b) The user has granted us both the Phone and Contacts permissions

Bug: 24270592

Change-Id: I232314e42f5331a72c1a84be917d9d74c5813976