nuplayer: Fix incorrect audio resume time during seek

GenericSource uses the timestamp of last video buffer
read in readBuffer() to seek audio, due to which audio
starts at a later position leading to frame drops.

Add change to use the first video sample timestamp
to seek audio

Change-Id: I5f2061e67994a3a0330b527bd3164ee04ed04005
CRs-Fixed: 780094