[MGit] Utilize `com.android.documentsui` as file picker

#### **Is your feature request related to a problem? <!-- Please describe. -->** <!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] --> Currently, it's custom: >  This prevents it accessing https://developer.android.com/training/data-storage/shared/documents-files, whereas utilizing the native FP would. #### **Describe the solution you'd like** <!-- A clear and concise description of what you want to happen. --> Utilize the native file picker (`com.android.documentsui`): >  #### **Describe alternatives you've considered** <!-- A clear and concise description of any alternative solutions or features you've considered. --> Unfortunately, the ability to access such virtual directories cannot be