Fix doc flag for device having multiple storages

Previously MtpDocumentsProvider#queryDocument returned
FLAG_DIR_SUPPORTS_CREATE for device having multiple storages.

This was wrong because if the device has multiple storages,
MtpDocumentsProvider places the storages under the device document, thus
users cannot create a document just under the device document.

Bug: 35700994
Test: MtpDocumentsProviderTests
Change-Id: Id73a34a2eaf4e10e23be3c2da7488036cea10000