We present X-Droid, a framework that provides Android app developers an ability to quickly and easily produce functional prototypes. Our work is motivated by the need for such ability and the lack of tools that provide it. Developers want to produce a functional prototype rapidly to test out potential features in real-life situations. However, current prototyping tools for mobile apps are limited to creating non-functional UI mockups that do not demonstrate actual features. With X-Droid, developers can create a new app that imports various kinds of functionality provided by other existing Android apps. In doing so, developers do not need to understand how other Android apps are implemented or need access to their source code. X-Droid provides a developer tool that enables developers to use the UIs of other Android apps and import desired functions into their prototypes. X-Droid also provides a run-time system that executes other apps' functionality in the background on off-the-shelf Android devices for seamless integration. Our evaluation shows that with the help of X-Droid, a developer imported a function from an existing Android app into a new prototype with only 51 lines of Java code, while the function itself requires 10, 334 lines of Java code to implement (i.e., 200× improvement).