Make a small correction in terms of GOP structure in the recorded video

When we have a 30 fps frame rate, and one second key-frame or I-frame interval,
we really would like to have for each second, 29 P-frames + 1 I-frame. Thus,
we should calculate the number of P frames so that it is equal to
frame_rate * I_frame_interval - 1

Change-Id: I5b9be6e4c101e7a6b718015aa4041496961c0f19