Fix "some user restrictions not working" issue

SettingsProvider used to prevent any changes to certain settings
when the corresponding user restriction is set, which isn't really what
these restrictions mean.
Even if a user restriction is set, it should still changing in the more
restricting direction.

Also stop setting "" to LOCATION_PROVIDERS_ALLOWED, which will simply
be ignored.

Bug 25614198

Change-Id: Ifa8edc2927e21e6c6174620c8c874c86c1dc0f75