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