[Lens] (Accessibility) Improve screen reader announcement of the layer settings button

[myasonik]

The problem

The layer settings button (which has the chart type picker popover) also visually indicates the chart type but requires extra navigating for screen reader users to get the same context.

The fix

Two possible ways to fix this:

• If you have it available somewhere on the page, could add an aria-describedby={idOfTheString} pointing to the chart type.
• If not, could extend the aria-label to read something like Edit layer settings; {chartType} (instead of the current Edit layer settings)

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[elasticmachine]

Pinging @elastic/kibana-accessibility (Project:Accessibility)