Implement ID mapping for root documents.

MTP identifiers like object handle is unstable over MTP sessions.  The
CL introduce two methods to MtpDatabase that enables the provider to
remap exisitng document ID with new MTP identifiers when the provider
reopen the MTP session.

 * MtpDatabase#clearMtpIdentifier invalidates old MTP identifiers
 * MtpDatabase#resolveRootDocuments remaps invalidated documents with
   new MTP identifier.

BUG=25162822

Change-Id: Iadb668ae5932da81aba2b2731d0d185da5656f16