[#320] NO_IMEI should be treated as null

On WiFi only mobile devices (no SIM, no imei) the dashboard was storing
"NO_IMEI" as value, and that makes all this devices to be handled as the
same.

If we try to get a device by `imei` and is "NO_IMEI" we immediatly
return `null`, and the related code needs to search by `phoneNumber`.

`phoneNumber` property stores either the actual mobile phone number or
the MAC address of the device.
[#320] Make private constant
[#320] Move constant to Device domain class, adapt related code

Move the `NO_IMEI` constant to Device domain class, and adapt related
code (the classes calling `DeviceDAO.getByImei`
Merge pull request #543 from akvo/feature/320

Feature/320