NuPlyer:RTSP: Fix some issues while seeking RTSP source

- Clear the obsolete packets after sending the seek request
- Stop rendering any obsolete frames while seeking
- Stop queueing any obsolete units to source while seeking
- Without this patch, the process-bar will return to
  the previous playback position, and stay at this position
  until the seek operation finish.
  Then, the process-bar will go to the seek position, and go
  forward according the playback of the clips.
- Without this patch, in some platform (based on Q6), seek
  operation will cause several green frame.

CRs-Fixed: 623269
Change-Id: I432de355d8e601a4d434cb78c699043dea2c6b55