[SB Refactor] Migrate the isWifiDefault bit to the new pipeline.

This migrates WifiStatusTracker's `isDefaultNetwork` tracking, and
WifiSignalController's `wifiVisible` calculation.

Bug: 238425913
Test: manual: Turn wifi off then back on, verify that wifi icon shows up
as soon as wifi becomes default again.
Test: statusbar.pipeline tests

Change-Id: Ice2bbb217e5c834ccef9905dca0ce47388775c67