Backup external files directory

Data from the external files directory (/sdcard/Android/data/) is backed
up if the directory exists.
Option to enable backup of external files
Handle compression in static methods
Add output directory to unzip method

This can be used to extract data directly to the external files
directory.
Restore external data

Unzips the external data directly to the externalFilesDir if the option
is enabled.
Name of external data backup directory as constant
Remember to swap path with external files

The code that uses cp to copy the external data needs
swapBackupDirPath but the part that zips doesn't need it.