dashplayer: Adjust buffer count for native window

When allocating buffers from native window, adjust the
buffer count based on the minimum undequed buffer count,
and allocate an extra buffer for smooth streaming.

Change-Id: I72ad33872b68bb0bfe41fcb565d61b429f76b892
Signed-off-by: Hieu Phan <aznrice2k4@gmail.com>