Add flag for close fullscreen/split-screen shortcut

This change introduces a feature flag to enable the Action+Ctrl+W
keyboard shortcut for closing fullscreen and split-screen tasks,
extending its functionality beyond desktop-only windows.

Bug: 441147192
Test: None
Flag: com.android.window.flags.close_fullscreen_and_splitscreen_keyboard_shortcut
Change-Id: I52bd209e57357ad60827096bd51cbcee603efa4d