Refactor background/system user behavior

This is a breaking change so that providers are no longer disabled for
non-current (ie background users). Instead, location requests from
background users are now considered inactive. This brings provider
behavior more in line with other APIs (such as GNSS and geofencing),
and allows us to special case requests from the system UID.

Without this change, the system UID would see incorrect provider
enabled/disabled notifications. Impact on clients is expected to be
minimal, as by far the vast majority of android applications run on
single user devices. For multiuser devices, applications will no longer
see enabled/disabled notifications in response to current user changes.

Bug: 157682495
Test: manual + presubmit
Change-Id: I0edb15f0792bf1426855baefcda8477ba3a32b28