Remove display ID association requirement for VirtualInputDevices

Allow no display ID associations, i.e. Display.INVALID_DISPLAY, so that
virtual input devices can target any display.

Bug: 422265772
Test: atest VirtualInputDeviceControllerTest

Flag: com.android.hardware.input.create_virtual_keyboard_api

Change-Id: Ie5e3a05ce86314d1e00e20cf30274b9cfa486d03