Fix incorrect aria description in List View

[talldan]

Description

Blocks in list view have an aria-description that helps screenreader users know the position they're at within the tree grid. A description like 'Block 1 of 1, level 1' is read.

Currently, there's a bug where the indexing is off by 1 resulting in the possibility of a message like 'Block 1 of 0, level 1', which doesn't make sense. I think this happened because there was a change to optionally showing the appender within the treegrid, but didn't update the row count to coexist with this logic.

This fixes the issue, the indexes should now be correct.

How has this been tested?

1. Add several blocks to a post with various levels of nesting
2. Using the keyboard and a screenreader, navigate around the List View.
3. Observe or hear that the aria description for each block is now correct.

Screenshots

Before:

After:

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[github-actions]

Size Change: -5 B (0%)

Total Size: 1.16 MB

Filename	Size	Change
build/block-editor/index.js	125 kB	-5 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.14 kB	0 B
build/annotations/index.js	3.67 kB	0 B
build/api-fetch/index.js	3.44 kB	0 B
build/autop/index.js	2.82 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	7.97 kB	0 B
build/block-directory/style-rtl.css	953 B	0 B
build/block-directory/style.css	952 B	0 B
build/block-editor/style-rtl.css	10.6 kB	0 B
build/block-editor/style.css	10.6 kB	0 B
build/block-library/editor-rtl.css	8.36 kB	0 B
build/block-library/editor.css	8.36 kB	0 B
build/block-library/index.js	132 kB	0 B
build/block-library/style-rtl.css	7.49 kB	0 B
build/block-library/style.css	7.49 kB	0 B
build/block-library/theme-rtl.css	729 B	0 B
build/block-library/theme.css	730 B	0 B
build/block-serialization-default-parser/index.js	1.88 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	48.4 kB	0 B
build/components/index.js	200 kB	0 B
build/components/style-rtl.css	15.7 kB	0 B
build/components/style.css	15.7 kB	0 B
build/compose/index.js	9.68 kB	0 B
build/core-data/index.js	11.8 kB	0 B
build/data-controls/index.js	1.29 kB	0 B
build/data/index.js	8.45 kB	0 B
build/date/index.js	5.38 kB	0 B
build/deprecated/index.js	772 B	0 B
build/dom-ready/index.js	568 B	0 B
build/dom/index.js	3.23 kB	0 B
build/edit-navigation/index.js	10.9 kB	0 B
build/edit-navigation/style-rtl.css	1.08 kB	0 B
build/edit-navigation/style.css	1.08 kB	0 B
build/edit-post/index.js	304 kB	0 B
build/edit-post/style-rtl.css	5.63 kB	0 B
build/edit-post/style.css	5.63 kB	0 B
build/edit-site/index.js	17 kB	0 B
build/edit-site/style-rtl.css	3.06 kB	0 B
build/edit-site/style.css	3.06 kB	0 B
build/edit-widgets/index.js	11.7 kB	0 B
build/edit-widgets/style-rtl.css	2.45 kB	0 B
build/edit-widgets/style.css	2.45 kB	0 B
build/editor/editor-styles-rtl.css	537 B	0 B
build/editor/editor-styles.css	539 B	0 B
build/editor/index.js	45.3 kB	0 B
build/editor/style-rtl.css	3.8 kB	0 B
build/editor/style.css	3.79 kB	0 B
build/element/index.js	4.65 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	7.72 kB	0 B
build/format-library/style-rtl.css	547 B	0 B
build/format-library/style.css	548 B	0 B
build/hooks/index.js	2.13 kB	0 B
build/html-entities/index.js	621 B	0 B
build/i18n/index.js	3.56 kB	0 B
build/is-shallow-equal/index.js	711 B	0 B
build/keyboard-shortcuts/index.js	2.52 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.11 kB	0 B
build/list-reusable-blocks/style-rtl.css	476 B	0 B
build/list-reusable-blocks/style.css	476 B	0 B
build/media-utils/index.js	5.33 kB	0 B
build/notices/index.js	1.79 kB	0 B
build/nux/index.js	3.4 kB	0 B
build/nux/style-rtl.css	671 B	0 B
build/nux/style.css	668 B	0 B
build/plugins/index.js	2.56 kB	0 B
build/primitives/index.js	1.41 kB	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.85 kB	0 B
build/rich-text/index.js	13.9 kB	0 B
build/server-side-render/index.js	2.71 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.27 kB	0 B
build/url/index.js	4.06 kB	0 B
build/viewport/index.js	1.85 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.17 kB	0 B

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[ntsekouras]

Other than the small comment, LGTM 👍