[WifiCandidates] Change Candidate to an interface

Add some abstraction here to make it easier to test a CandidateScorer in
isolation.

Remove CandidateScorerExample from tests, it is now redundant.

Bug: 124023173
Test: atest FrameworksWifiTests
Change-Id: I0e3da551292d30b030c5cbf65eddf501e526e7dc
7 files changed