Fix handling of current_parent* driven default value expression when editing a child feature

[nirvn]

This PR fixes #7668

Looking into the issue revealed a couple of issues this PR addresses:

• we had a poorly designed method to check for the usage of a parent feature driving default value expressions; and
• we did not refresh parent feature driven default value expressions when opening a child feature in editing mode.

I've also taken the time to deduplicate code here to ease maintenance and improvements to child editing.

[qfield-fairy]

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