Stealing of sensitive information from apps is always considered to be one of the most critical threats to Android security. Recent studies show that this can happen even to the apps without explicit implementation flaws, through exploiting some design weaknesses of the operating system, e.g., shared communication channels such as audio and Bluetooth, and side channels like CPU, memory, network-data usages, etc. In all these attacks, a malicious app needs to run side-by-side with the target app (the victim) to collect its runtime information. Examples include recording phone conversations from the phone app, gathering network-data usages of WebMD to infer the disease condition the user looks at, etc. This runtime-information-gathering (RIG) threat is both realistic and serious, as demonstrated by prior research and our new findings, which reveal that the adversary monitoring daily operations of popular Android-based home security systems can easily figure out when the house is empty and the user is not looking at surveillance cameras, and even turn off the alarm delivered to the user's phone. To defend against this new category of attacks, we propose a novel technique that changes neither the operating system nor the target apps, and provides immediate protection as soon as an ordinary app (with only normal and dangerous permissions) is installed. This new approach, called App Guardian, thwarts a malicious app's runtime monitoring attempt by pausing all suspicious background processes when the target app (called principal) is running in the foreground, and resuming them after the app stops and its runtime environment is cleaned up. Our technique leverages a unique feature of Android, on which third-party apps running in the background are often considered to be disposable and can be stopped anytime with only a minor performance and utility implication. We further limit such an impact by only focusing on a small set of suspicious background apps, which are identified based upon their behaviors inferred from their side channels, such as thread names, CPU scheduling data and kernel time. App Guardian is also carefully designed to choose the right moments to start and end the protection procedure, and effectively protect itself against malicious apps. Our experimental studies show that this new technique defeated all known RIG attacks, ranging from phone taping to keylogging through various side channels. In the meantime, the inconvenience it introduces is found to be minimal, with negligible impacts on the utility of legitimate apps and the performance of the OS.