Map picker workaround for private folder

[moving-bits]

Description

Since more recent versions of Android WhereYouGo can no longer access public folders to load Mapsforge map files.
WhereYouGo can still read its private folder in Android/data/menion.whereyougo.com/files, but the internal file picker cannot reach that folder.

This PR allows resetting the starting point for the internal file picker to either "root" (as it is currently) or "private" folder. It also resets the internally stored last map file name, so that WhereYouGo resorts to the (reset) default folder, effectively allowing the user to select map files stored in WhereYouGo private folder.

Users are able to put map files into that private folder by connecting their mobile to a PC and transferring the map file from there to the mentioned private folder.

Additional context

new preferences	confirmation	select map from private folder

[Lineflyer]

@moving-bits Can we do something similar for the WIG-folder?

[moving-bits]

@moving-bits Can we do something similar for the WIG-folder?

For cartridges, you can go to settings => global => Whereigo folder and select "default". Then WhereYouGo will find all cartridges downloaded by itself after that.

The "trick" for maps will not help with storing maps in a user-created folder, same goes for cartridges. They will not be visible to WhereYouGo without SAF, when using more recent Android versions.