imho the best android app i've seen is cross DJ pro. its created by Mixvibes - the same guys who built Rekordbox. it does key analysis, sync, effects, records your mixes and, yes, retains a history of tracks you've played. I use it on the train on the way to work on a cheap 7inch tablet I picked up at JB-hifi for $99 bucks. It will work on most android phones, but i highly recommend for use on one that has a larger screen to get the full benefit.