Extend close window shortcut to fullscreen tasks

This change extends the Action+Ctrl+W keyboard shortcut to close the
focused fullscreen task. This builds upon the existing functionality
that allows closing desktop windows.

The feature is controlled by the
`close_fullscreen_and_splitscreen_keyboard_shortcut` flag.

Bug: 441147192
Test: DesktopModeKeyGestureHandlerTest, DesktopModeWindowDecorViewModelTests
Flag: com.android.window.flags.close_fullscreen_and_splitscreen_keyboard_shortcut
Change-Id: Icc934c009a53e5b33a9b8363a456b4c5c68e66a3