Performances fixes

[iskugor]

The problem is ".nomedia" file is being search through many directories, this solution caches directories that has been already checked.

Description

The solution uses HashSet to cache directories which has been traversed already.

Motivation and Context

https://github.com/jaiselrahman/FilePicker/issues/57

How Has This Been Tested?

Tested on Samsung S10 with a lot of images and other files

Screenshots (if appropriate):

Types of changes

• [x ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.