Utilize Qt global discovery mode session in QBluetootLocalDevice.

QtBluezDiscoveryManager manges the global discovery session for all Qt
classes. This was introduced when implementing the device discovery.
QBluetoothLocalDevice was not using this mechanism yet which could lead
to discovery sessions being killed while still required.

Change-Id: I4229cb2aa34df35cf4d0650c828c8cfa11040d4a
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Code cleanup: Avoid repeated service info duplication check

Change-Id: I00701e9103a6acfdfc2697949280f8a7de8bcdfe
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>
Fix crash during call to QBluetoothServiceDiscoveryAgent::stop()

This was triggered when the stop() call was made while the SDP thread
was talking to the SDP server.

Change-Id: If6624dce21ca4a099d5beae064685b6b3a907989
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>