Change the inserter search result message from assertive to polite

[afercia]

This PR seeks to improve screen readers behavior when using the Inserter. For more details please refer to the related issue #13387.

Please first test on current master:

• use Safari + VoiceOver
• tab to the main inserter button
• press Control Option Spacebar
• the inserter opens, focus is moved to the search field
• the search field label "Search for a block" is not announced
• the number of results is announced almost immediately

Switch to this branch

• repeats the steps above
• check the search field label "Search for a block" is announced correctly, followed by the number of results

Fixes #13387

[afercia]

Rebased to incorporate the changes from #13395 and pass the tests.

[jorgefilipecosta]

I'm not sure of the reason why a message using 'assertive' option disappears faster, but I followed the test steps, and I verified that before the message disappeared almost immediately on voice over without being totally spoken and now the message is completely spoken.

[afercia]

Thanks @jorgefilipecosta. To clarify: it's the assertive message "61 results found" that was interrupting the screen reader while it was reading the search field label, so "Search for a block" wasn't announced.

With this change, the search field label is read out first, followed by the number of results.