Avoid turning off wifi tethering on interface down

The hostapd daemon can bring down an interface, followed by immediately bringing it up
and this results in us turning off tethering completely.

Wait for the right notification to enter tethered state after brining up hostapd.

Bug: 5135245
Change-Id: I95c8c0c4c2c99c03fce2d18c014eaff37ed4e762