Thread fixes for WifiStatusTracker tests

WifiStatusTracker now supports sending messages to the background. Tests
were adapted to make sure threading is taken into account.

Test: atest NetworkControllerDataTest
Test: atest NetworkControllerSignalTest
Test: atest NetworkControllerWifiTest
Bug: 190376958
Change-Id: Id80bba6a2abec36a2e58a81552a34a7f74f9a818