overlay: overlay the overlay with moar overlays

On dlx, we need to overlay our own verizon eri.xml's because these break
other devices (namely toro) by causing a roaming icon. Ours does the
opposite if we don't have them.

Also need to switch to using PRODUCT_PACKAGE_OVERLAYS instead of
DEVICE_PACKAGE_OVERLAYS and inherit it before vendor/cm/ overlays or it won't
take effect.

Change-Id: I792d6f05cc47029425c73b82afb82dc07d92f6dc
Merge "overlay: overlay the overlay with moar overlays" into cm-11.0