libstagefright: Change allocation buffer count for VideoEditor

- OMXCodec: AllocateBuffersOnPort: Add a check to allocated
  minmum buffer count for encoder, min + 1 for decoder input
  and min + 2 for decoder output

Change-Id: I483cbe0587b76b0c029e11054eda666e548c2b63

Conflicts:

	include/media/stagefright/OMXCodec.h