For some reason, when running html5 applications in mobile devices' browsers, the UserMedia plugin always answers NO to "Supports user media" condition. It doesn't matter which browser you use, the condition is always false, so it seems "impossible" to use UserMedia features in mobile devices.
However, this can be solved by exporting the project as a native app with cordova. If you do so, the UserMedia works well.
If you just want to record the whole screen, there is another alternative to UserMedia. You could use the Snapshot Canvas system action https://www.scirra.com/manual/125/system-actions capturing a screen shot every 0.X seconds.