Implement key gesture to close focused desktop task

This change adds the handling for the
`QUIT_FOCUSED_DESKTOP_TASK` key gesture, allowing users to close the
currently focused window with a keyboard shortcut.

The task closing logic in `DesktopModeWindowDecorViewModel` has been
refactored into a common method to be shared between the window's
close button and this new key gesture.

Bug: 416437920
Test: DesktopModeKeyGestureHandlerTest
Test: DesktopModeWindowDecorViewModelTests
Flag: com.android.window.flags.close_task_keyboard_shortcut
Change-Id: I9338f71d80f7068f3d0de09d800df3e1cc0e8ad4