[SB Refactor] Make some wifi pipeline classes into interfaces.

This is needed for WifiScreenshotTest so that more of the pipeline can
be faked out.

Bug: 238425913
Test: manual: verified new wifi icon still works
Test: atest WifiScreenshotTest
Test: atest all tests in statusbar.pipeline
Change-Id: I335451e62c5db01143226d7cdc3ee29b93301bf2