Keep VirtualDevice-created VirtualDisplays awake

When the owner of a VirtualDevice creates a VirtualDisplay, grab a
display-specific wakelock for the lifetime of the VirtualDisplay to keep
it awake.

Test: atest VirtualDeviceManagerServiceTest
Fixes: 214334661
Change-Id: I4ba14c2004023efeda8c011e5dae7fce04b0746e