Reset wifi tethering state when failing in QS.

Call CM#stopTethering upon soft AP failure state if tethering in enabled
through QS. Also, clean up code by relying on SoftApCallback instead of
wifi AP broadcasts.

Change-Id: I93686c66247681fbfd950d6a9dabab435033475c
Merged-In: I93686c66247681fbfd950d6a9dabab435033475c
Fixes: 110303540
Test: manual