How can I regulate the framerate in my android app? I would like the game to run at a constant speed. My app doesn't require a high framerate so I don't want one because that would take up more battery then necessary.
Don't use frame rate to measure time. Use time to measure time. A GC pass can take 3/10 of a second, other tasks can fire up in the background, etc etc.
There's always a system/setup that will run slower than you thought possible.
Then you don't specify velocities in pixels, but in pixels/second. Each frame of animation takes a certain amount of time, etc etc. In your game engine, when computing the next frame, one of your inputs is "how much time has passed since the last frame". You determine that value as a fraction of a second, and multiply your velocity/sec by that fraction. The result is how far a Given Thing has moved since the last frame.
Note that really slow frame rates can wreak havoc on collision detection, particularly with fast moving objects. If Thing 1 passes completely through Thing 2 between frames, just checking BBoxes or radii isn't going to cut it.
Having said all that, sleep() is your friend. At the start of a frame's processing, call System.currentTimeMilis(). At the end of a frame's processing (including rendering), check the current time again. If the difference isn't long enough, sleep(N) for enough time to match your desired frame rate.
So if you want 20fps max, then each frame should take 50ms (1000ms / 20 = 50ms). If a frame only took 10ms to simulate and render, then you need to sleep for another (50ms - 10ms = ) 40ms before moving on to the next frame.
Alternatively, you can keep running the simulation as fast as possible, and only render the screen every so often. This won't help battery life much (though OpenGL hardware acceleration is expensive if the heat coming off my Evo is any indication), but can make for a Very Smooth experience. Heck, you can start calculating things like motion blur at that point.