Expose Android's Environment DIRECTORY_ fields

I'd like to be able to list the contents of some of Android's common directories (Documents, Downloads, etc.) in my app. These directories are available natively through the Environment object's fields (https://developer.android.com/reference/android/os/Environment.html), but don't appear to be exposed through cordova-plugin-file.

It would be a huge boon to expose these as cordova.file.* fields for Android (I'm guessing they would return null values on other platforms).