Fix crash on Wi-Fi Slice when it's continuously pinned and unpinned

It's because WifiTracker's initialization and onStop is on different
thread. Fine tune the thread logic in SliceBackgroundWorker.

Fixes: 118165942
Test: manual
Change-Id: Icc86b5df7ec3c6fd0e4a79a62ea0c84465e9528d
4 files changed