libstagefright: Reduce number of video events posted when frame is early

Current av sync logic posts an early video frame with a delay of 10 ms.
This may result in multiple video events being posted before the frame
gets rendered. Update the delay for posting an early frame to the difference
between the lateness and early margin.

Change-Id: I619b75cf1b2ed83ab9e07bf26ad759d7c03fb4f4