[CD Taskbar] Stop External Taskbar Destruction on Window Close

When all windows on external taskbar are closed, exit desktop mode is trigggered. We stop this signal from destroying the external taskbar. This is because we want to be in desktop mode as long as we are connected to a non-mirroring display.

Flag: com.android.window.flags.enable_taskbar_connected_displays
Bug: 401553128
Test: m
Change-Id: I23b61172bb13f1377c4532a56e7838fbe2140f20