Show MTP device as root when it has multiple/zero storages.

The CL updates MtpDocumentsProvider#queryRoots so that it fetches
devices from Database as well as storages when we don't have storages
under the device, or when we have multiple storages under the device.

BUG=26120019
Change-Id: Id2b140f00a1d49fa4da7e17d2564dbbaa1795e1e
7 files changed