stagefright: correctly calculate and configure P, B and I-frame spacing

- though OMX spec says otherwise, vendors' implementation of nBFrames
  is # of B frames between I/P frames.
- nPFrames is # of P frames between I frames.
  thus, I-frame interval (nKeyFrameInterval) is
       (nPFrames + 1) * nBFrames + nPFrames + 1 =
        nPFrames * nBFrames + nBFrames + nPFrames + 1

     I   P   I  : I-interval: 8, nPFrames 1, nBFrames 3
      BBB BBB

- err on lower P and B-frames values as key-frame-interval is an upper limit.

Bug: 30096321
Change-Id: I1d0c2908007a1a283f36a7c19e85b95465236c7d