How to use FFmpeg Command for Reverse Video?
FFmpeg has a reverse video filter and reverse audio filter.
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

This filter buffers the entire clip. For larger files, segment the file, reverse each segment and then concat the reversed segments. Remember that you have to concat the reversed segments in the reverse order!