docs: public static void setRegisteredOnServer(Context context, boolean flag) {
          final SharedPreferences prefs = getGCMPreferences(context);
          Editor editor = prefs.edit();
          editor.putBoolean(PROPERTY_ON_SERVER, flag);
          // set the flag's expiration date
          long lifespan = getRegisterOnServerLifespan(context);
          long expirationTime = System.currentTimeMillis() + lifespan;
          Log.v(TAG, "Setting registeredOnServer status as " + flag + " until " +
                  new Timestamp(expirationTime));
          editor.putLong(PROPERTY_ON_SERVER_EXPIRATION_TIME, expirationTime);
          editor.commit();
      }