Fix up kotlin files for footer inlining

This is to avoid ktfmt-related noise in the follow-up CL to inline the
footer refactor flag. Also took the opportunity to change affected tests
to use mockito-kotlin, and fix some minor warnings.

Bug: 375010573
Test: builds, tests pass
Flag: TEST_ONLY
Flag: EXEMPT formatting only change
Change-Id: I812f09ef098b1c61a5bc430a18320f9ee2b1e276