Remove the dependency on Mockito to create PeopleSpace fakes (1/2)

This CL refactors PeopleViewModel so that it can be created easily with
fake values in the Gallery app and screenshot tests. That way, we don't
need to depend on Mockito to create fake dependencies of the ViewModel,
which is necessary for deviceless RNG tests (and better architecture)
and Compose @Previews.

This is a pure refactoring that doesn't change any logic in the code.

Bug: 304719047
Test: atest PeopleSpaceScreenshotTest
Change-Id: I701a2fa6b5dd933c9be9d6b0d97deb62b75ba3d8