Add keyboard shortcut to close focused task

This CL introduces a keyboard shortcut (Action+Ctrl+W) to close the
currently focused task, in addtion to the existing long press ESC. This
provides users with a quick way to dismiss applications using the
keyboard.

The feature is controlled by the `close_task_keyboard_shortcut` flag.

Bug: 416437920
Test: adb shell am start -n com.google.android.calculator/com.android.calculator2.Calculator && sleep 2 && adb shell input keycombination KEYCODE_META_LEFT KEYCODE_CTRL_LEFT KEYCODE_W
Flag: com.android.window.flags.close_task_keyboard_shortcut
Change-Id: Ief8dbc45f592bdd4f2b9b2c212c6deb824d736ae