ConnectivityService uses SettingsObservers to detect when
settings change. These are all set on user zero. This is fine
for most settings, which are in Settings.Global, but some
settings are per-user, such as MOBILE_DATA_PREFERRED_UIDS, which
is in Settings.Secure.
This CL adds the ability to register SettingsObservers on all
users. These observers will fire any time the setting changes
for any current user, and any time a user is added or removed.
This CL does not add or change any SettingsObservers and should
cause no behaviour changes.
Bug: 404186833
Flag: EXEMPT refactor
Test: covered by existing tests
Change-Id: I5954b196eea776414098f1ce4441eb02065438ef
aad9267493