[Sb refactor] Pull the WifiRepository interface out of the impl file (1/2)

In order to keep the git commit history cleaner, this commit duplicates
the file WifiRepository.kt, and moves the interface definition from one
to the other. The follow-up to this renames the Impl's file

Test: existing tests
Bug: 238425913
Change-Id: I40a6d5aa7e1c5e82954c35a233ca23e8942ac6f3