Unfortunately, no products exist as of this moment, for high-quality stereo recording on Android. It's a real shame... I've been looking for something similar.
However, there is a very good chance that your phone does have an audio input as a secondary microphone input within the headphone jack. If you are just getting recordings to practice with and review, the quality is plenty good for this. Also note that this input is mono only.
Step 1: The Hardware
You need a 3.5mm TRRS cable. This cable is the normal size of a small headphone plug, but has an extra conductor. If you have a camcorder, you probably have one of these cables around. If not, you can pick them up online for
Your microphone input will generally appear on the red RCA lead. Next, you need to bring that line-level output from a mixer down to a microphone level. The cheapest way to do this is a 40dB pad. You can buy them online or just build one yourself easily with two resistors.
From there, you just need whatever plug adapters necessary to hook up your RCA cable to your pad, and then to your mixer. (Alternatively, what I have done is cut off all the plugs, and fitted my pad resistors and wire directly inside of a plug, for a nice clean and easy cable.)
Step 2: The Software
There are many applications available in the Google Play store for recording. Not just any software will do. You need one that supports recording from an alternative microphone. I found RecForge Pro awhile back to be suitable. I did not do extensive experimentation with software to see which is best. Please shop around and find one that works for you.
The feature you are looking for is something along the lines of "recording from external mic", but it also might be labeled as "regular mic and camcorder mic". The reason is, the Android SDK exposes the microphone inputs as
DEFAULT. Which one applies to your external input is device specific, so it may take some experimentation.
Step 3: Recording
That's all there is to it! Just remember to not drive the device into clipping. Also, not all devices have the same audio quality. Some are much better than others. If you have bad luck, try using someone else's phone. Finally, experiment with turning off the automatic gain control. It has a tendency to make recordings sound bad.
Best of luck!