Fix off-by-1 errors in saving video settings when in a playlists.

Current video settings are saved in:
  * `PlayFile()` before playing the requested (next) item - this works for playlists;
  * `UpdateFileState()` every 500 ms (it is called in `ProcessSlow()`), but only if the
    current playing item has changed - this works for single videos.

Due to `SaveFileState()` calling `GetCurrentVideoSettings()` [1], in a playlist
the correct video settings were first saved by PlayFile(), but shortly after
overwritten by UpdateFileState().  It was in fact saving video settings for
the *previous* item (X-1) with settings for the *current* one (X) - see _Did the file change?_.

This commits adds a check to UpdateFileState() so that it ignores playlists.

Fixes #15322

[1] https://github.com/xbmc/xbmc/blob/master/xbmc/Application.cpp#L4350