Update JavaDoc in MtpDatabase.

 * Database for MTP objects.

 * The object handle which is identifier for object in MTP protocol is not stable over sessions.

 * When we resume the process, we need to remap our document ID with MTP's object handle.

 * The database object remembers the map of document ID and fullpath, and helps to remap object

 * handle and document ID by comparing fullpath.

 *

 * 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 artificial ID generated by MtpDatabase itself. The database object

 * remembers the map of document ID and object handle, and remaps new object handle with document ID

 * by comparing the directory structure and object name.

 *

 * TODO: Remove @VisibleForTesting annotation when we start to use this class.

 */

@VisibleForTesting