Add "Close active window" to shortcut helper

This CL adds the "Close active window" shortcut (Meta+Ctrl+W) to the
keyboard shortcut helper screen. This makes the feature, which was
added in a previous CL, discoverable to users.

Bug: 416437920
Test: Manually trigger shortcut helper (Meta+/) and verify the "Close
active window" shortcut is listed.
Flag: com.android.window.flags.close_task_keyboard_shortcut

Change-Id: I479a4bd14d9aebf3732cc117f3c6f88d4fb46061