Updater: Allow Updater to handle major upgrades via property

* If the user has GMS installed, they'll need to install an updated
  copy, as addon.d won't restore it, and if it did it likely wouldn't
  work. So we can't support this unconditionally.
* If the user has no-GMS, this should be totally fine to do, so the
  user could set the property, upgrade, and go on their merry way.
  This use-case is why I didn't make this an `ro.` property.
* If someone building LineageOS builds GMS into the images, upgrade
  via this method would also be feasible.

Change-Id: If2a6e6767dcb182d3dbd2a95c81a940fdb53547e