src: Fix deadlock when issuing mouse input via virtual touchpad

The MirAL documentation says to avoid running code within invoke_under_lock when called by a thread that calls WindowManagementPolicy methods, which we are effectively are doing here.

Docs: https://mir-server.io/doc/classmiral_1_1_window_manager_tools.html#a5c0dabb1ac759c8ac8ceec879bfe8b75

Remove the use of invoke_under_lock to fix deadlocks when using the Lomiri shell in desktop mode via Aethercast.