Hi,
Not all Mp4 vids work in iMovie. iMovie likes Mp4/AAC. There may be some codec in Android's Mp4 format that iMovie does not like.
Convert the clip by running it through the free download, Handbrake. Handbrake will by default convert it to an Mp4 format that works in iMovie.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to
complete. Then save and import the converted clip into iMovie.
-- Rich