Viva Video is the most popular and free video editing smartphone app that's available today. You can cut, edit, insert texts, insert music of your choice and even put effects to your video. You can download several themes as well, like birthdays, travels, love, dynamic, etc. This app is very easy to use. I think you should try this one.
Unfortunately, there will be a "Made with Viva Video" watermark that's going to appear to your video once you finished editing one.
Like this. The one that's located at the lower right corner.
Good thing is that there is a site where you can download "Viva Video Pro" for free, where you will not see a watermark. I don't remember where I downloaded it, but I suggest you to Google it. Start searching through torrent sites.
Here is the link of
Free Viva Video.