make FLP play nicely with multiuser

Frameworks' FusedLocationProvider runs as a specific user so that it
can join a specific process. The solution that works for NLP, run one
copy per user as that user, does not work for FLP.

To make FLP play nicely with multiuser, I've allowed SYSTEM_UID to
operate in the background and included a hardcoded exception to
prevent ServiceWatcher from trying to launch one FLP per user.

Bug: 7279799
Change-Id: I573ea5226d8d00777421b39c5c3fb0899bf09b4d