Allow overriding of displayId on Task.TaskKey

Test: launch apps on VDM display from VDM host (use ./development/samples/VirtualDeviceManager/setup.sh -s and 'single device mode' in settings)
Test: open Overview
Test: change policy in VDM host settings and observe tasks on default display
Flag: EXEMPT bugfix
Bug: 437848012

Change-Id: I40f40eb54d7fdb13254161f96f0f65c83dd144c9