Add e2e test for close task keyboard shortcut

This change adds an end-to-end test to verify that a focused desktop
task can be closed using the `Meta+Ctrl+W` keyboard shortcut.

The test launches an app in desktop mode, sends the key combination,
and asserts that the app window is closed.

Bug: 416437920
Test: CloseTaskViaKeyboardShortcutTest
Flag: com.android.window.flags.close_task_keyboard_shortcut
Change-Id: I50cfd54e8ffaea86688b0186fcbcf36269379a3a