Flat UI is pretty popular these days. Now, not long after Bootstrap for Android became an option, there is another tool kit to help you keep your apps looking as up-to-date as they can: The Flat UI Kit from CengaLabs. It's all available on their GitHub, and it's basically what you'd expect from a flat UI kit - just all neatly packaged for you in one place. Here are some examples:
Check out the GitHub for more details, and give the Flat UI Kit a try if you're looking for an easier way to implement a clean, flat design.