Fix race conditions in starting adb

Since we don't return the start-server exec promise, clients would run
more adb commands before the server was up. This would cause "Connection
Refused" to be returned to the second adb command, and everyone was
confused.

Actually returning the exec allows downstreams to wait on the server
starting.

Ref: ubports/ubports-installer#1847

src/adb.js

@@ -82,9 +82,7 @@ export class Adb extends Tool {
    * @returns {Promise}
    */
   startServer() {
-    return this.killServer().then(() => {
-      this.exec("start-server");
-    });
+    return this.killServer().then(() => this.exec("start-server"));
   }
 
   /**