Allow showing content on virtual displays when locked

This adds new flag to virtual display that changes its
behavior when keyguard is shown, but can be dismissed without
entering credentials. This can be applied only to private
virtual displays.

Bug: 34280365
Test: android.server.cts.ActivityManagerDisplayTests
Test: #testVirtualDisplayHidesContentWhenLocked
Test: #testShowWhenLockedVirtualDisplay
Test: #testShowWhenLockedPublicVirtualDisplay
Change-Id: I9c19d36295e62f59bd1db1352af707d54ea51667