Provide one preferred way (ideally with DI) of providing test address books

[rfc2822]

Are they even necessary or can normal LocalAddressBookss used?

In any case, the code that manages test address books should be in a Hilt (→ no Context passing) class, be it LocalAddressBookStore or an extension of it or a separate one.

So:

• Think about better class/test design (including testing philosophy: mock vs fake class vs real class).
• Decide whether LocalTestAddressBook is necessary.
• Check subclasses vs interfaces vs extension methods.