Remove importantForA11y=no from a11y activity launch preference.

Due to how Preferences create a single node for their content, this was
causing that node to lack the ACTION_CLICK action and clickable
attribute.

Removing this attribute properly exposes the click action which means
that TalkBack will now speak "double tap to activate" and Switch Access
will highlight the button while scanning.
See http://recall/-/bznujeSAo20YqWFB1N28dO/gVdpAXoD3BroEkUeJBTUWq

Flag: RESOURCES_ONLY
Bug: 301528206
Test: Open Settings > Accessibility > Sound Notifications;
      Use TalkBack to highlight and speak the "Open Sound Notifications"
      button; observe "double tap to activate".
      Repeat with Switch Access; observe button is highlighted while
      scanning and can be selected.
Change-Id: Ie747e19fe149d015cf92fbe07fd71cfaf183d795