overlay: overlay the overlay with moar overlays

We need to overlay our own verizon eri.xml's these break other devices (namely toro)
by causing a roaming icon. Without them, we get "Roaming Indicator Off" instead of
"Verizon Wireless". Which I'm fine with, but most end users stab me over it.

Need to switch to using PRODUCT_PACKAGES_OVERLAY instead of DEVICE_PACKAGES_OVERLAY
or it won't take precedence.

Change-Id: Ieba26464ae1872644de213544bd970a9cdd46efc