add one feature and fix some issues to file manager app

I have a file manager app with following requirments: 1. load files faster preventing "app is not responding " [login to view URL] going back to last screen on pressing back button, fix crash problems(if any) [login to view URL] for sd card access, if not granted already, first time when user wants to perform a write operation on sd card files(delete,rename, move, copy etc) Note: the project is being created on API 17 and I want to get it done using methods which does not require me to change API level of the project