Settings: Add switch preference for AGPS

Allow toggling AGPS (which in turn disables SUPL) using the existing
AOSP mechanism that was simply never exposed to the user.
Based on the following commits from CodeAurora, modified to
use SwitchPreference and improved strings:

Author: juwei <juwei@codeaurora.org>
Date:   Mon May 7 13:31:50 2018 +0800
    Settings: Add AGPS setting menu.

    Add AGPS setting switch to allow user turn on/off
    AGPS function.

    Change-Id: If8a20a8293b9f4b12933bc3820b4cd135a938b72
    CRs-Fixed: 2169854

Author: hoffc <hoffc@codeaurora.org>
Date:   Wed Dec 19 15:56:07 2018 +0800
    Settings: Refactor Agps preference controller code

    (1) Extract inner class AgpsPreferenceController as one standalone file.
    (2) Delete the useless code of reading the SUPL server and port from etc/gps.conf file.
    (3) Remove the config item "config_agps_enabled".

    Change-Id: I20465aa4375c7fe64bc84f579a39178506b8e211
    CRs-Fixed: 2370304

Author: hoffc <hoffc@codeaurora.org>
Date:   Tue Jun 1 08:52:31 2021 +0800
    Settings: fix NPE caused by missing agps preference definition

    No agps preference is defined in location settings, results in NPE when agps controller init

    add agps preference definition in location settings

    Change-Id: I92e3293bccb90da7b6a1057e2dd83f8aa909d308
    CRs-Fixed: 2958241

Issue: calyxos#1490
Change-Id: Idea7026f0c954c1f2a056e7b4595a879ef161f5c