Bug 1537723 Enable LWT updates on Android r=mixedpuppy

Also ensure that LightweightThemeManager.updateOneTheme() returns
even if the update request fails.

Differential Revision: https://phabricator.services.mozilla.com/D24981

--HG--
extra : moz-landing-system : lando

mobile/android/app/mobile.js

@@ -198,6 +198,7 @@ pref("extensions.autoupdate.enabled", true);
 pref("extensions.autoupdate.interval", 86400);
 pref("extensions.update.enabled", true);
 pref("extensions.update.interval", 86400);
+pref("lightweightThemes.update.enabled", true);
 pref("extensions.dss.enabled", false);
 pref("extensions.ignoreMTimeChanges", false);
 pref("extensions.logging.enabled", false);

toolkit/mozapps/extensions/LightweightThemeManager.jsm

@@ -303,6 +303,7 @@ var LightweightThemeManager = {
 
     await new Promise(resolve => {
       req.addEventListener("load", resolve, {once: true});
+      req.addEventListener("error", resolve, {once: true});
       req.send(null);
     });