[WifiTrackerLib] Return SPEED_NONE if ScoredNetwork has no speed info

The speed value of a ScoredNetwork should be interpreted as SPEED_NONE
if the value is 0, instead of being bucketed into SPEED_SLOW.

Bug: 155846675
Test: build
Change-Id: I6e234f6888c2283f90cf8b7aa8debe6325292505
1 file changed