Fix compilation with -Wall -Werror

Relates NEMO#699
[BluetoothDevicesModel] Notice existing adapters

Relates NEMO#699
[BluetoothDevicesModel] Handle DefaultAdapterChanged

Relates NEMO#699
Add 'coverage' make target

Relates NEMO#699
[tests] Initial commit

Relates NEMO#699
ut_agent: Initial commit

Relates NEMO#699
ut_bluetoothdevicesmodel: Initial commit