Make HotspotControllerImpl async

HotspotControllerImpl now listens to events from TetheringManager to
determine if hotspot is supported.

Also, switch to TetheringManager API because ConnectivityManager is
deprecated.

Test: atest
Fixes: 147949379
Fixes: 146821053
Change-Id: If9abfb1239140030ed303214c9953826df374591