Remove fullpath from MtpDatabase.

If the remote MTP device is backed by typical file system, the file name
is unique among files in a directory. However, MTP protocol itself does
not guarantee the uniqueness of name so we cannot use fullpath as ID.

Instead of fullpath, we use artifical ID generated by MtpDatabase
itself. So we don't need to store fullpath in the database.

BUG=25162822

Change-Id: I06598ce631a3221ed72e11734dbdaefef4c6349c