Added callback from Tethering to PanService

To remove hidden networking APIs we need a new way of
communication with Tethering. Adds callback to notify
Tethering when Bluetooth is ready.

Tag: #feature
Bug: 190438212
Test: atest BluetoothInstrumentationTests
Change-Id: I3102d7c907f9933fceab9da35c2e83982878ba82