WifiOverlay: Don't sign Wifi RROs with platform cert

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

rro_overlays/WifiOverlay/Android.bp

@@ -1,7 +1,6 @@
 runtime_resource_overlay {
     name: "WifiOverlay",
     theme: "WifiOverlay",
-    certificate: "platform",
     sdk_version: "current",
     product_specific: true
 }