code: public static void startAppOnPebble(final Context context, final UUID watchappUuid)
              throws IllegalArgumentException {
  
          if (watchappUuid == null) {
              throw new IllegalArgumentException("uuid cannot be null");
          }
  
          final Intent startAppIntent = new Intent(INTENT_APP_START);
          startAppIntent.putExtra(APP_UUID, watchappUuid);
          context.sendBroadcast(startAppIntent);
      }
docs: Send a message to the connected Pebble to launch an application identified by a UUID. If another application is
  currently running it will be terminated and the new application will be brought to the foreground.
  
  @param context
  The context used to send the broadcast.
  @param watchappUuid
  A UUID uniquely identifying the target application. UUIDs for the stock PebbleKit applications are
  available in {@link Constants}.
  
  @throws IllegalArgumentException
  Thrown if the specified UUID is invalid.