fix "always need to unlock gesture pattern when changing library" error

Though the error was fixed, there still left another serious problem here, which damages user experience.

After set up Gesture Lock, it cannot restore its checked state, so every time resume back to Settings menu, the Gesture Lock checkbox was always unchecked.

This bug will be fixed in next PR.
这里实现得有点复杂了。比较简单的方法是：

SettingsManager 里有一个时间戳 T，当用户成功解锁时，将时间戳 T 的值更新为当前时间。
下次需要解锁时，对比当前时间与 T 的值，如果两者相差在 5 分钟以内，就说明不用再次解锁，否则需要解锁。