feat(empty-cards): Add 'keep notes' checkbox

[david-allison]

Purpose / Description

Hey all, I found a possible improvement to avoid some data loss as of 2.20.0.

In Anki desktop, the Empty Cards feature asks if it should not only delete cards that are empty, but also associated notes that would remain without any valid cards.

On AnkiDroid, this is on by default. Notes with no valid cards get deleted when I press the Empty Cards.

I noticed because I made blank placeholder notes that I wanted to later fill in with questions & answers.

It'd be nice if the mobile version also asked first

Fixes

• Issue Improve the Empty cards dialog #17213

Approach

• Extract to a DialogFragment
• Implement the feature using a custom layout and a checkbox
• Fixup the dialog a bit (Material theme + fix title)

How Has This Been Tested?

Unit tested, somewhat

Learning (optional, can help others)

• A custom view is needed if you want a message AND checkboxes

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[mikehardy]

one question use of Bundle as message passing mechanism in face of possibly unbounded data to pass, otherwise LGTM

[lukstbit]

LGTM, with a note for the error state when searching the cards.