[WifiTrackerLib] Show "Low quality" only if cell outscores wifi

"Low quality" text for a wifi entry should only show if wifi is
validated but the default route is still cellular, which indicates that
the score of wifi is insufficiently low.

Bug: 163627176
Test: atest WifiTrackerLibTests
Change-Id: I1c1f9218354274214222dd6ed92e6b451d74e3f4
12 files changed