Audio Evolution Mobile DEMO is a multi-track recording studio for Android that supports more than 200 USB/MIDI interfaces and virtual instruments. It includes quite interesting features like a non-linear editing bar with unlimited undo/redo options. You can also apply real-time sound effects over your recordings.
With Audio Evolution Mobile DEMO you can import and export audio files to WAV, MP3, AIFF, FLAC, and OGG/Vorbis. Whichever format you're working with, you can benefit from all the features on the app and use as many tracks as you want (it supports unlimited tracks).
It's important to note that this version of the app is a demo. This means that although you can use many of its features, you can't save your projects. Again: with this version you CANNOT save your projects.
Audio Evolution Mobile DEMO is one of the most comprehensive recording studio apps out there. The only inconvenience, if it can be considered one, is that it's a paid app, and the demo version definitely makes you want to buy it.