floss: Handle register_disconnect for btmanagerd

Handle dbus client disconnection in btmanagerd using the
register_disconnect RPCProxy method. This makes sure that old callbacks
that are no longer connected get removed once the dbus client is gone.

Bug: 193061373
Test: Debug log in btmanagerd and open + quit btclient
Tag: #floss
Change-Id: Id26bbbb20def9332f5e3704c88a862ee901008df