Make virtual displays never blank only if in default group

If a private non-mirror virtual display is not in the default
group (either its own or in a virtual device group), then it
already doesn't share the power state with the default display.

So we should not treat such displays as never blank because then
they can never be turned off, even if their power group is off.

Fix: 433644644
Bug: 421917324
Test: presubmit
Flag: EXEMPT bugfix
Change-Id: I41b8dba8efa3c03ad908bd25be5861e58eced628