How do I reverse a video with ffmpeg?
For video only:

ffmpeg -i /storage/emulated/0/ffvid/frameCount.mp4 -vf reverse reversed.mp4

For audio and video:

ffmpeg -i /storage/emulated/0/ffvid/frameCount.mp4 -vf reverse -af areverse reversed.mp4

Source: https://video.stackexchange.com/questions/17738/how-to-use-ffmpeg-command-for-reverse-video