Drop device name from storage document name.

When device has multiple storages, storage are shown as directory under
the device root. In this case, we would not like to add device name to
storage.

Note that we still use "device name + storage name" for root name if the
device has a single storage because we skip storage directory in this
case and shows storage's contents directly under the device.

BUG=26625708

Change-Id: Ie13b044e71ae9b5131c4a01ee9d605023d05f168