Add ability for daemons to subscribe to pairing session changes

Added Service::SetPairingInfoListener() to provide a callback to
be invoked when pairing session starts/ends. This will let vendor
daemons to display/indicate pairing code to the user.

BUG: 26467076
Change-Id: Ic531c6599eb0dc665670346e5df59cb2cb3ed4cf
2 files changed