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
28da2e3490