public static ContentStorage get() {
        return INSTANCE;
    }
Central class to interact with locally stored Folders.
<br>
Encapsulates and unifies access to the Android SAF framework as well as the File framework
(and maybe later also the MediaStore API?)
<br>
Note that methods of this class do not and cannot ask user for access permissions when dealing with SAF This can only be
done in context of an Activity (using Intents) and is encapsulated in
{@link ContentStorageActivityHelper}.
When dealing with {@link PersistableFolder}s, methods of this class will try to fall back to default
accessible folders when a folder is not accessible.
<br>
Implementation reference(s) with regards to SAF:
* Android Doku: <a href="https://developer.android.com/preview/privacy/storage">...</a>
* Android Doku on use cases: <a href="https://developer.android.com/training/data-storage/use-cases#handle-non-media-files">...</a>
* Introduction: <a href="https://www.androidcentral.com/what-scoped-storage">...</a>
* Helpers: <a href="https://stackoverflow.com/questions/34927748/android-5-0-documentfile-from-tree-uri">...</a>
main/src/main/java/cgeo/geocaching/storage/ContentStorage.java