Use public API of Settings.Secure to get/put secure settings

Get/put secure settings by using public API of Settings.Secure to
prevent using hidden API.

Bug: 171033200
Test: atest FrameworksNetTests
Test: Install 3rd-party VPN app and create multiple users, then
      change the VPN always-on setting in one of users and check
      if the VPN always-on setting of the others is not changed.
      (i.e. Make sure they won't influence each other.)
Change-Id: Icd455df3fa6eac52cdab1b162556f429da99e8f7