Address TaskFragmentOrganizer style comments

1. Rename onActivityReparentToTask to onActivityReparentedToTask
2. Annotate @NonNull and @Nullable for the Extensions methods that are
   not yet annotated.

Bug: 240519866
Test: Refactor and add annotation. Pass existing tests.
Change-Id: I85c5927e6e7261cae3bce715b1955523d430e1e2