[Tether] Prefer mergedCarrierEntry() over connectedEntry()

WifiPickerTracker does not return a MergedCarrierWifiEntry as the
connectedEntry() if it exists. Clients instead must specifically check
for that type of wifi network first.

This wasn't caught before because the tests used a mock
WifiPickerTracker that didn't exhibit the same characteristics.

Test: WifiRepositoryViaTrackerLibTest
Bug: 301333393
Change-Id: I265d573d2603e1d749dd301403cd7ac36aa21b92