Fix green or corrupted video frames in the exported movies

At least two use cases are affected: trimming and transition clip generation.

If the starting time of the transition or the trimming position is not the same as
that of any existing video frame, the video frame right before the transition
starting position or the trimming position must be encoded as the the first video
frame for the transtion or the trimmed video, and it is encoded as a key frame.
This frame was not saved without the patch.

In the transcoding phrase, since we could not find such a frame, the existing
code did not return an error, and incorrectly used some other frame to serve
as the reference key frame for the remaining video frames of the transition or
the trimmed video. As a result, we may see corrupted or green video frames in
the exported movies.

The fix is to always save the frame right before the trimming position or
the transition starting time, so that a key frame will always be encoded, and
the right reference frame is used for remaining video frames. In addition,
checks are added to prevent from future mistakes to use incorrect reference frames.

Patch was originally contributed by teng.hong@nxp.com

Change-Id: I182784d4c1786224445224634edab3338bf2278f
related-to-bug: 5713461