fw/b: [Squash] Add capability to allow tethering to use VPN upstreams

Squash of:

Author: Sam Mortimer <sam@mortimer.me.uk>
Date:   Thu Aug 15 19:40:55 2019 -0700

    fw/b: Add capability to allow tethering to use VPN upstreams

    * Toggled on/off at runtime via a new hotspot lineage setting.

    * Dynamically updates the tethering upstream for existing hotspot
      clients as VPNs are brought up / down or the hotspot setting
      is changed.

    * This implementation depends on fw/b config_tether_upstream_automatic
      being set to true.

    Change-Id: I2ac0b4acc0ea686dfdf54561cb3428808e337160

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Author: Sam Mortimer <sam@mortimer.me.uk>
Date:   Fri Sep 13 16:27:37 2019 -0700

    fw/b: Prevent trying to enable hw offload for tethering via VPN upstreams

    * Tethering via VPN upstream requires a sw path.

    * hw offload setup happened to be being disabled anyway owing to a fail
      return code from setDataLimit().  However, it was causing offload to be
      disabled entirely (until next hotspot off / on event).

    * Gracefully skip hw offload for vpn upstreams so that it is automatically
      used again when a vpn is disconnected.

    Change-Id: I4df13f02889305560903b7b1e919eedc7af78c07

Change-Id: I77ed0142e653f4993486eea44e4dac21e3f67f17