Tethering: Fix number of device connected

The following commit prevents notification from being shown again when
notification is the same as previous one.

db3fe9ed Fix Tethering Notifications for
multiple ifaces

This is true except for the Wifi tethering which can have multiple
connections. Patch taken from cm12.1 code.

Change-Id: Ic30c6d3498595e2ba0561c709789fd6db1113f69