Add ContactIdWrapper interface and implementations

...lin/org/briarproject/briar/desktop/contact/ContactCard.kt

...rg/briarproject/briar/desktop/contact/ContactIdWrapper.kt

...lin/org/briarproject/briar/desktop/contact/ContactItem.kt

...lin/org/briarproject/briar/desktop/contact/ContactList.kt

...riarproject/briar/desktop/contact/ContactListViewModel.kt

...g/briarproject/briar/desktop/contact/ContactsViewModel.kt

...rproject/briar/desktop/contact/PendingContactIdWrapper.kt

.../briarproject/briar/desktop/contact/PendingContactItem.kt

...riarproject/briar/desktop/contact/RealContactIdWrapper.kt

...org/briarproject/briar/desktop/contact/RealContactItem.kt

...roject/briar/desktop/conversation/PrivateMessageScreen.kt

...oject/briar/desktop/introduction/IntroductionViewModel.kt