Bug 943825 - [Gaia] Mount/Unmount SD card

User story for SD card management.
If storages.length is not a changing value I'd like to put the get this._storage part in constructor. The line before https://github.com/mozilla-b2g/gaia/pull/16981/files#diff-6873e1605ae29958a1e9990ef3aeef4eR187