Prevent NPE in OverviewProxyService's LauncherStateChangedReceiver

It's possible that there's no package that receives QUICKSTEP_SERVICE
intent. In this case, PackageManager#resolveService returns null.
(Actually the method is annotated as nullable.)

This adds an explicit null check in the corresponding code.

Flag: none
Bug: 324510233
Test: tree hugger.
Test: ARC++ no longer throws NPE.
Change-Id: I2046d8bfca98963f58ff537b5769579c7dd6a49c