WifiNetworkSelector: Allow default CandidateScorer

Allow a CandidateScorer to be used as the default.

This CL leaves the legacy scorer active. A follow-on will
use CompatibilityScorer as the default.

Also, the active CandidateScorer may now choose "none of the above".

Bug: 128937950
Bug: 126273496
Test: atest FrameworksWifiTests
Change-Id: Ia518e7c8a9d383de59985aa186de1e4fab990054
2 files changed