[SB][Wifi] Create a dedicated thread for WifiTrackerLib.

WifiTrackerLib operations can take 10-20s on startup. If those
operations use the background handler, then it blocks all other
background operations because we currently only have 1 thread for all
background handler tasks. This creates a separate thread for
WifiTrackerLib so it doesn't block other operations.

Fixes: 299571835
Test: restart device & quickly open QS -> verify tiles load correctly
Test: atest AccessPointControllerImplTest
WifiRepositoryViaTrackerLibTest

Change-Id: I4900fc61b9151f63a775d9a1c49d3dbcafca1d87