At the moment, for version 5.1.5 and earlier versions, it wasn’t possible to have ffmpeg running on Android.
The 5.2 Release Candidate was intended to have ffmpeg baked in but there are teething problems:
Krita 5.2 Release Candidate is out! Please help us in another round of testing! - #12 by abhifx
A formal release of version 5.2 shouldn’t be too far away now so I suggest that you wait for that happy event.
Or, you could be brave and decide to get the 5.2 apk running on your tablet to help with the testing 
This forum has many people who use krita on an Android tablet. Do a search for ‘Android’ or ‘tablet’ or ‘samsung’ or ‘galaxy’ with the search tool at the top of the main page.