code: public String getDefaultApp(Set<String> supportedApps) {
      String defaultApp = getAut();
      // if the launch activity is specified, just return.
      if (getLaunchActivity() != null) {
        return defaultApp;
      }
      // App version is not specified. Get the latest version from the apps store.
      if (!defaultApp.contains(":")) {
        return getDefaultVersion(supportedApps, defaultApp);
      }
      return supportedApps.contains(defaultApp) ? defaultApp : null;
    }
docs: Returns the application under test in the format of "appName:appVersion", or "appName" if the supported application
  does not have any version associated with it, or returns null if the requested app is not in the apps store. If the
  launch activity is also specified with requested application then just return the requested application as app under
  test so it can be later installed to the device by SelendroidStandaloneDriver.
  
  @param supportedApps The list of supported apps in the apps store.
  @return The application under test in "appName" or "appName:appVersion" format, or null if the application is not
  in the list of supported apps and the launch activity is not specified.