Bug 948992 - Give deviceSerial number to ADB's device manager.

This transfer the device_serial of Marionette to the DeviceManagerADB which is using it to prefix all adb commands with the "-s <deviceSerial>" command line arguments.  Without this, one might not be able to run gaia-ui-tests if multiple devices are plugged on the computer.

This branch should only be merged as soon as the Gecko's part of it has landed.  Otherwise the function get_device_manager will not exists.