MOBILE-3556 Login: Send extra parameter on token.php check

The extra parameter can be used by Moodle to avoid throwing an error
in server logs because other parameters e.g. username are not supplied.

src/providers/sites.ts

@@ -641,7 +641,8 @@ export class CoreSitesProvider {
      * @return A promise to be resolved if the site exists.
      */
     siteExists(siteUrl: string): Promise<void> {
-        return this.http.post(siteUrl + '/login/token.php', {}).timeout(this.wsProvider.getRequestTimeout()).toPromise()
+        return this.http.post(siteUrl + '/login/token.php', { appsitecheck: 1 }).
+                timeout(this.wsProvider.getRequestTimeout()).toPromise()
                 .catch(() => {
             // Default error messages are kinda bad, return our own message.
             return Promise.reject({error: this.translate.instant('core.cannotconnecttrouble')});