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 a lot.

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.
We should record the last time when the user has successfully provided the correct gesture lock, and do not ask the user to unlock within several minutes (say 2 minutes).