Android: Allow listContentURI to list content of a subfolder

[MaxGyver83]

Description:

Assume this file structure, p.e. in /sdcard/Download/:

folder
├── a
└── subfolder
    └── b

When you list the content of folder, you get a and subfolder (as expected). But when you then try to list the content of folder/subfolder, you get a and subfolder again instead of b.

This pull request fixes this.

Test program:

package main

import (
	"fmt"
	"net/url"

	"fyne.io/fyne/v2"
	"fyne.io/fyne/v2/app"
	"fyne.io/fyne/v2/container"
	"fyne.io/fyne/v2/dialog"
	"fyne.io/fyne/v2/storage"
	"fyne.io/fyne/v2/widget"
)

var w fyne.Window

func main() {
	a := app.New()
	w = a.NewWindow("List content of subdirectory")
	btn := widget.NewButton("Open folder", selectFolder)
	w.SetContent(container.NewVBox(btn))
	w.ShowAndRun()
}

func selectFolder() {
	dialog.ShowFolderOpen(func(u fyne.ListableURI, err error) {
		if err != nil || u == nil {
			return
		}
		openFolder(u)
	}, w)
}

func openFolder(u fyne.URI) {
	fmt.Println("Open folder", u.String())
	filelist, err := storage.List(u)
	if err != nil {
		fmt.Println(err)
		return
	}

	for _, file := range filelist {
		name, _ := url.PathUnescape(file.Name())
		fmt.Println("-", name)
		if isdir, err := storage.CanList(file); err == nil && isdir {
			filelist, err := storage.List(file)
			if err == nil {
				for _, file := range filelist {
					name, _ = url.PathUnescape(file.Name())
					fmt.Println("  -", name)
				}
			}
		}
	}
}

Output on develop:

04-03 15:09:22.286 23985 25311 I Fyne    : - primary:Download/folder/a
04-03 15:09:22.299 23985 25311 I Fyne    : - primary:Download/folder/subfolder
04-03 15:09:22.327 23985 25311 I Fyne    :   - primary:Download/folder/a
04-03 15:09:22.327 23985 25311 I Fyne    :   - primary:Download/folder/subfolder

With this PR:

04-03 15:11:02.795 24825 25891 I Fyne    : - primary:Download/folder/a
04-03 15:11:02.803 24825 25891 I Fyne    : - primary:Download/folder/subfolder
04-03 15:11:02.826 24825 25891 I Fyne    :   - primary:Download/folder/subfolder/b

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.