libstagefright : Allow seek in RTSP VOD streaming using Helix Server

- In the current design,when the server sends npt=0- in Play response,
we treat it as live stream, without cross verifying in SDP.
- Helix server specifically sends npt=0- for the Play Response
- This change cross verifies the npt in SDP and if it is a VOD
clip, the duration returned is greater than Zero, and we allow seek.

Change-Id: Ic76510fa4d80ef9051a5d6d923bd5d757c279f19
CRs-Fixed: 498057