GnssLocationProvider: Disable AGPS by default, it'll be enabled in SetupWizard

* This way the user can choose to turn it off, and it'll
  never have been enabled
* If they just keep it as the default (toggle will be on in SuW),
  then it'll be enabled, which is the default we want

Change-Id: I878200a0351d0b0018a975e432602de2badc4eef