Run WifiScanner callback on worker thread

WifiScanner callbacks are currently running on the main thread, which
causes concurrent modification exceptions and duplicated networks in the
wifi picker. Make sure to post these callbacks to the worker thread
instead.

Bug: 277168460
Test: Manually verify wifi picker doesn't crash due to concurrent
modification exceptions, and that duplicate networks aren't showing up.

Change-Id: I3ff50ee7546d510f435b26c0ebbf4b2486e7f99a
3 files changed