Apps and their integrated third party libraries often collect a variety of data from people to show them personalized ads. This practice is often privacy-invasive. Since 2013, Google has therefore allowed users to limit ad tracking on Android via system settings. Further, under the 2018 California Consumer Privacy Act (CCPA), apps must honor opt-outs from ad tracking under the Global Privacy Control (GPC). The efficacy of these two methods to limit ad tracking has not been studied in prior work. Our legal and technical analysis details how the GPC applies to mobile apps and how it could be integrated directly into Android, thereby developing a reference design for GPC on Android. Our empirical analysis of 1,896 top-ranked Android apps shows that both the Android system-level opt-out and the GPC signal rarely restrict ad tracking. In our view, deleting the AdID and opting out under the CCPA has the same meaning. Thus, the current AdID setting and APIs should be evolved towards GPC and integrated into Android's Privacy Sandbox.