[video_player_avplay] Add start position in player options when creating player

[xiaowei-guan]

Add start position in player options when creating player,this is useful for resuming playback from last viewed position.
code sample:

    _controller = VideoPlayerController.network(
        'https://media.w3.org/2010/05/bunny/trailer.mp4',
         // startPosition in millisecond
        playerOptions: {'startPosition': 10 * 1000});

[JSUYA]

It seems that the condition of the state calling the Seek() API could be kIdle or higher. The state condition of SeekTo() could also be modified to avoid confusion.