[Linux] add missing AtkObject::get_index_in_parent implementation

[jpnurmi]

Due to missing AtkObject::get_index_in_parent implementation, the Orca screen reader would lose track of accessibility objects and they would turn to "zombies".

Fixes: flutter/flutter#101513
See also: https://gitlab.gnome.org/GNOME/orca/-/blob/master/src/orca/script_utilities.py#L5041

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[cbracken]

@jpnurmi can you think of a way we can write a test for this?

[jpnurmi]

Sure, I'll come up with something 👍

[jpnurmi]

I'm glad you asked me to add a test because it made me realize that the first fix attempt wasn't really correct. 🙂

[cbracken]

LGTM!

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[cbracken]

Added @chunhtai for second review.

[chunhtai]

LGTM