Synchronize access to mCurrentState when updating

WifiSignalController expects broadcast updates to come in off the main
thread, and WifiStatusTracker updates to come in on the main thread.

This change ensures data access synchronization by making sure that all
data access happens on a known background thread.

Bug: 229425925
Test: atest NetworkControllerWifiTest
Change-Id: Ifc04b2e25b0cc08ad6406030fa9de3df0ba2df2d