Fix getPojavStorageRoot handling

[thejudge156]

Android spec dictates that getExternalFilesDir can return null.

Our getPojavStorageRoot will crash relying methods if getExternalFilesDir returns null, add handling for said case.

Notes:

• Since this is the only shared storage location we can try, and we can't do shared, fall back to private directory.
• Adds a marker file to indicate to the app that files are stored privately in case getExternalFilesDir's state changes later.

[alexytomi]

Did you test whether the app actually works in private dir? Afaik there are a lot of strewn across assumptions using Android/data

[thejudge156]

Did you test whether the app actually works in private dir? Afaik there are a lot of strewn across assumptions using Android/data

This only takes effect on Android >=29, I just double-checked and there aren't any references that should fire post Android 29.

I did not test it, as I do not have a device that can test that.

[thejudge156]

Made a quick commit, just made the marker file name a constant.