Add a onClientsChanged callback to OnTetheringEventCallback.
The callback will provide information on connected clients combining
at least DHCP leases and WiFi AP information (WiFi AP tethering used).
Test: atest TetheringTests
Bug: 135411507
Change-Id: I7065d081c11bc606d691f76ac8b499dd075d6504
Merged-In: I7065d081c11bc606d691f76ac8b499dd075d6504