remember the subtitle file last time choosed

tower: https://tower.im/projects/d4e0ff64158645f28d99a9ca72d0c27b/todos/677e9489f87e459199f7b591a0e4e12d
tower: https://tower.im/projects/d4e0ff64158645f28d99a9ca72d0c27b/todos/9314d340152b4135a459d11f02719f76

Change-Id: I4ee5f3927b6dc8f13cab82d088f55dda74ab6867