enableTetheringInternal immediately sends the tether result for all
types except for TETHERING_BLUETOOTH, which relies on
setBluetoothTethering to sends the result immediately if the Pan
service is connected, or defer the result until after the Pan service
connects.
To keep better track of the active listeners and simplify the logic for
further listener behavior changes (i.e. notifying the listener after the
link layer event), refactor the bluetooth listener to be called
immediately within enableTetheringInternal if the Pan service is
connected like the other tethering types.
Bug: 216524590
Test: covered by existing tests
Change-Id: I91196e430b44ef2391de662ff0ed6e051b453d98
48fbdf4a36