Enable certificate overrides with product vars.

Currently it is only for android_app, though it can be easily ported to
apex.

The make-side change will be made later, along with a real application.

Bug: 122957760
Test: app_test.go
Change-Id: I41f0be84f8b9f93e9518a16160e10eaa649388cd

android/variable.go

@@ -270,6 +270,7 @@ type productVariables struct {
 	DexpreoptGlobalConfig *string `json:",omitempty"`
 
 	ManifestPackageNameOverrides []string `json:",omitempty"`
+	CertificateOverrides         []string `json:",omitempty"`
 
 	EnforceSystemCertificate          *bool    `json:",omitempty"`
 	EnforceSystemCertificateWhitelist []string `json:",omitempty"`