Clear calling identity in StorageManagerService#onAppopsChanged

It turns out StorageManagerService#onAppopsChanged runs with the
calling identity of the app setting the uid.

We should run with system_server calling identity before calling into
ActivityManagerService to kill an app when appops changed

Test: PackageInstaller doesn't get killed
Bug: 149895648
Change-Id: I584b1ee340203d8653941fe325ff9bb01ec04ab6