The cache folder size returned by react-native-fs is always wrong

[umbopepato]

Ref itinance/react-native-fs#884 (comment).

[lpezzolla]

The size returned by the library should be divided by 1000 (it is actually documented as bytes, but used as kb in our code).

On Android, the size differs slightly from the sum of contained files as also the size of the folder is considered, but it does not bother me.

On iOS it seems to return the size of the inode (which is clearly useless) - increased by 32 bytes for each downloaded file into the folder.

The size difference between the single course and the settings screen is caused by the mixed usage of the root folder (it is also used to cache network responses and other RN stuff) - we might want to introduce a containing folder for all course files.