nuplayer: Fix incorrect audio timestamp at EOS

For few clips the EOS buffer is received with size as 0 bytes
and timestamp as 0. NuPlayerRenderer updates the mAnchorMediaTime
without validating the timestamp and henceforth getCurrentPosition
returns incorrect position. This results a huge time stamp jump in
video frames and the renderer keeps checking every 10msec whether
the video frame can be rendered or not.

Do not update the audio mAnchorMediaTime if the buffer is EOS buffer
and empty.

CRs-Fixed: 771960

Change-Id: Ic6f2f3684ea9842f01c9189362a86b216df8df37