Add OS.get_external_data_dir() to get Android external directory

[madmiraal]

Android provides each app with a dedicated location for external storage. However, this was often ignored because any external storage location could be accessed with the READ_EXTERNAL_STORAGE and WRITE_EXTERNAL_STORAGE permissions. With Android 10+, with scoped storage, the dedicated external storage location is the only external location that the app is allowed to access (without using the MediaStore API or requesting the MANAGE_EXTERNAL_STORAGE permission). On Android 10, this could be worked around with android:requestLegacyExternalStorage="true" (see #39103), however this workaround is disabled with Android 11 (see #47954).

This PR provides users with an easy way to locate the dedicated external storage location e.g.:

func _on_Save_Button_pressed():
	var dir = OS.get_external_data_dir()
	var image = $TextureRect.get_texture().get_data()
	image.save_png(dir + "/test.png")

To provide compatibility, on other OSs, this will return the same location as OS.get_user_data_dir().

Fixes #39414.
Should help with #38913 and #48636.

[m4gr3d]

Looks pretty good, thanks for adding the function! I just add one minor feedback to address, and it should be good for approval!

[akien-mga]

Thanks!

[m4gr3d]

fyi @akien-mga @madmiraal I'm likely to revert this change and just make get_user_data_dir() behave as expected on Android 10 and higher.

[madmiraal]

I'm likely to revert this change and just make get_user_data_dir() behave as expected on Android 10 and higher.

This is one of the many requirements to support Android's scoped storage. It should not be reverted. See my detailed review for more information.