Bug 1059546 - Set proper mozApps signed origins for eng builds r=rchien

Engineering builds needs to accept signed apps from stage Marketplace
for testing purpose. This depends on Gecko having a proper
setting-to-pref mapping for dom.mozApps.signed_apps_installable_from.
Merge pull request #24510 from lissyx/bug1059546

Bug 1059546 - Set proper mozApps signed origins for eng builds. r=rchien