Adjustments to target grid a11y hierarchical presentation.

Present subgrous as a ListView instead of GridView which removes
unnecessary row count announcement.
Mark the app list divider as unimportant for accessibility to make it
not taking a11y focus.

Fix: 380211084
Test: Manual testing
Flag: android.service.chooser.announce_shortcuts_and_suggested_apps_legacy
Change-Id: Ie5e07dbab7894ffc35a2d9f12d30a0da50d412e8