Free disk space left on phone (while downloading new Tileset) notification is inaccurate

[BirdSamuel]

Before downloading a new Tileset, Arbiter will ask you if it's okay to download with a before/after free disk space - to ensure that you have enough space on the SD Card. This number is not accurate, needs to be revisited.

[BirdSamuel]

Tested on phone, and it is accurate. However, I don't have external storage (SD card) on my phone. It correctly calculated my internal free space left.