Reordered the listener and install() invocation to avoid possible race

conditions and converted functions to async functions.

recipe-client-addon/lib/NormandyDriver.jsm

@@ -161,24 +161,25 @@ this.NormandyDriver = function(sandboxManager) {
      */
     async installAddon(installUrl) {
       const installObj = await AddonManager.getInstallForURL(installUrl, null, "application/x-xpinstall");
-      installObj.install();
-      return new Promise((resolve, reject) => installObj.addListener({
+      const result = new Promise((resolve, reject) => installObj.addListener({
         onInstallEnded(addonInstall, addon) {
           resolve(addon.id);
         },
         onInstallFailed(addonInstall) {
           reject(`AddonInstall error code: [${addonInstall.error}]`);
         },
       }));
+      installObj.install();
+      return result;
     },
 
     /*
      * Return a promise that resolves to a success messsage if
      * addon uninstall is successful.
      */
-    uninstallAddon(addonId) {
+    async uninstallAddon(addonId) {
       return (id => {
-        return new Promise(function(resolve, reject) {
+        return new Promise((resolve, reject) => {
           AddonManager.getAddonByID(id, addon => {
             if (addon !== null) {
               try {