Micro-controller to develop application for Android
The easiest way to get to an Android phone is to connect to it using Bluetooth. This can pretty much be as easy or hard as you want to make it. We have plenty of questions regarding bluetooth, but I think this one is probably the best for you.

As far as this community goes, we will probably be able to help the most with Arduino and Microchip, but anything that can support bluetooth can interact with Android phones.