Fix boot time regression

Spin up a new thread to perform Wifi initialization.

Also reordered initialization so that WifiScanningService
starts before WifiService, since WifiService needs to
call WifiScanner.

Bug: 141003116
Bug: 140938772

Test: atest google/perf/boottime/boottime-test -v
Change-Id: Idcc910f853a2270924bf138b636a3c56158cf1af