Annotate ActivityStarter as @NonNull

This change annotates ActivityStarter as @NonNull when creating a
dialog inside CreateUserDialogController and EditUserInfoController.

This is necessary because ActivityStarter gets dereferenced inside
EditUserPhotoController constructor.

Bug: 358375267
Test: This is an internal cleanup to resolve a Lint warning. All
existing tests still pass.
Flag: EXEMPT bugfix_only

Change-Id: I62e893ce083edea000c372f346f17b5be2222c2a