Percussion panel - additional keyboard navigation requirements

Overview

This issue outlines 3 additional requirements for the percussion panel in relation to keyboard navigation/accessibility.

1. Toolbar navigation

Original report by @avvvvve:

Pressing the left arrow when focusing within certain groups won't cycle back to the last focusable thing. i.e. focus on the Write/Preview button. Pressing the right, up, or down arrows over and over will swap between the two options, but pressing left when on "Write" does nothing. Same problem for 'Layout'/'Customize kit'.

Whoever investigates this (probably me) will need to consider whether this can/should be addressed at the percussion panel level, or if it's something that needs to be addressed at a higher level in our navigation system.

2. Pad navigation

Original report by @avvvvve:

Pressing up while focused in the middle of the top row moves focus left rather than back to the last row. Vice versa for the bottom row or a pad with empty slots beneath it: when pressing down, focus moves right when it should move down to the next row or back to the first one if you're already in the last row.

Same goes for this one, W.R.T. solving it at the navigation system level.

3. Screen reader behaviour (completed in #26728)

As outlined here by @shoogle, the screen reader should give some more detailed information when navigating pads (and footers). A design decision is needed here regarding the exact information to include.

Checklist

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