Request sync when there's no mediaItem in a mediaSet.

Request sync when there's no mediaItem in a mediaSet.

This is to fix the problem where media items haven't been sync'ed when the album
set or album is viewed for the first time.

+ Add MediaSet.SyncListener.
+ Make AlbumPage and AlbumSetPage implement SyncListener.
+ Implement requestSync() for ComboAlbum and ComboAlbumSet.
+ add ActivityState.isDestroyed(). This also fixes the problem where
  StateManager.finishState() may be called twice.

Bug: 5337899
Change-Id: I25364c3ac25721a2650701c5d7931bfb6daa9303