Add visitUri method to Person and MessagingStyle.Message.

This is both to avoid repeating code when visiting these classes, as
well as to make it more likely that if someone changes the class (e.g.
adds another icon), they see this method and update it accordingly.
Otherwise they'd have to know about Notification.visitUris.

Bug: 281044385
Test: atest NotificationTest NotificationManagerTest
Change-Id: I1ce6bebd9452466d005505dc5b99a0fdc0e05e80