Extend close window shortcut to split-screen tasks

This change extends the Action+Ctrl+W keyboard shortcut to close the
focused task in a split-screen pair. This builds upon the existing
functionality that allows closing desktop and fullscreen 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: Ibb9758db29714c02fdc785a5b93b2d9f27a56b52