This app was mentioned in 2 comments, with an average of 1.00 upvote
I've tried tons & the one I've had a lot of success with is Video Converter Beta despite its mediocre reviews. It's also the only one I've found that's been able to correct aspect ratios, which is how I stumbled upon it in the first place. Just make sure to install the proper Codec if you decide to try the app out.
I would also highly recommend spending the $2 for the pro version to unlock some useful features.
Also, converting gifs to webms has worked flawlessly & has proven to be a huge space saver as well.
This Android app is great b/c it will remix without actually transcoding. Should be able to side load it in the shield.
https://play.google.com/store/apps/details?id=com.akingi.tc.vbeta