This behavior is as designed. OnZoomListener only gets invoked for zoom in/out and pinch. We currently do not have a map extent changed event exposed in the Android API. This is coming with 10.3.
I hope this helps!
Ok, we're using workaround here, just call MapView.zoomin() and MapView.zoomout() at once. It triggers zoom event and seems not to affect performance at least on Nexus 10, and some Samsung tablets. As soon as new version of SDK gets available we get rid of this clumsy method.