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Don't register multiple UpdateEngine callbacks
We are loosely tied to UpdateEngine and have no way to know whether we are bound or not without keeping track of the connection manually. Since UpdaterService is for both A/B and legacy updates, turn ABUpdateInstaller into a singleton to keep the code simple while ensuring that a single callback is registered. Change-Id: Ib4e9ad1413ba96bf5ed59cc3383741b5c9bac427