Preinstalled overlays needn't be signed with the same cert
as the package it overlays, simply being preinstalled is
enough. Sign with the default cert instead for now, which
provides fewer special privileges.
Bug: 162195999
Test: verify values still overlaid correctly
Change-Id: I017491fd383551c5aadf8648edb900e0c12a27b8
Check device variant by using ro.boot.hwc and
ro.boot.proudct.hardware.sku, and set the correct model based on
value of ro.product.marketname on stock ROM
