Bluez5: Add service (un)registration support

The Bluez5 DBus API does not permit the selection of the
local Bt adapter when registering a service. This means that
if the local device has two Bluetooth adapters the service will be
advertised by both adapters. This leads to cases where
the QBluetoothServer::listen() call binds the socket on one adapter
but the remote device tries to connect to the other adapter.

For now this is an acceptable behavior since it is rather rare to
have two local adapters at the same time. A potential future fix is
tracked by QTBUG-39012.

Task-number: QTBUG-32085
Change-Id: Ibb24edc5b9d1c25a005aedd98f6fc5d0d11b60f8
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>
Bluez5: Simplify lookup of default adapter

Change-Id: I59aa5241d33191a8ca8cc167263b63b024f52754
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>