DashPlayer: Notify seek position to dash source

Notify seek position to dash source to make sure that
if text samples arrive before audio/video, the time
out for sending text samples is not wrongly calculated.

CRs-Fixed: 492376

Change-Id: I952e474d92936532956c1d456b6ad1d554402405
Signed-off-by: Hieu Phan <aznrice2k4@gmail.com>