Bug 929306 - [Gaia][Bluetooth File Transfer] Multiple file transfer UI

* IAC: Post message from Bluetooth app to System::BluetoothTransfer module while sending files.
* Maintain a sending files queue for summarise a report while user sending multiple files.
* Create a status bar icon for files sending/receiving.
* Because UX want to show utility tray while user clicked "Check the progress in Notification center" notification. Migrate it from Bluetooth app to System::BluetoothTransfer with adding callback function `UtilityTray.show();`

Un-completely section:
* Wait visual for status bar icon sprites.
* Adding unit test.