Move logic to skip single storage.

The tree structure of MTP model looks like /device/storage/objects. But
almost all MTP device has only single storage, so it's redundant to show
a single storage as a child of device in UI.

MtpDocumentsProvider has a special logic to skip single storage, and
shows storage's object as a children of device in such case. Previously
the logic was applied when MtpDocumentsProvider returned a root
list. The provider returns a storage as a Documents.Root, instead of
device if the device has only one storage.

However the number of root cannot be obtain for closed device. Thus the
previous logic did not work for closed devices that have a single
storage. The CL moves the logic from queryRoot to
queryChildDocuments. Now MtpDocumentsProvider always returns a device as
root, then it returns storage's objects as the device's children, where
we has already opened the device.

BUG=26481574

Change-Id: I25af0fc220410e321a378d67f226798ec4bba19c