Updates documentation for MessagingStyle constructor

MessagingStyle expects the field userReplyName to be non-null, but the
documentation doesn't describe it as such. This updates the documentation
to say the field is required, and adds a NonNull annotation.

This has no behavior changes.

BUG:31747744
Change-Id: If832d059c276e856fba366dabfa8a5821bb63054
1 file changed