gstplay: fix seek failure for rtsp stream

1. gstplay send seek event after the pipeline changes to paused.
2. But rtspsrc can't start playing in seek operation if it's paused. And then the audio/video sink can't finish preroll which cause failure.
3. There are two ways to slove this issue. One way is to send seek event in playing status instead of paused status in gstplay. The other way is to support playing in paused status after seek in rtspsrc.
4. This patch is to support seek in playing state only for live stream in gstplay.
5. The issue and the detail log can refer to #3502