Allow VDM virtual displays to get FLAG_OWN_DISPLAY_GROUP

For virtual displays that are created through VDM, allow
FLAG_OWN_DISPLAY_GROUP to be added. This applies to every display
created through VDM, because VDM adds this flag on behalf of the caller
by default.

Bug: 194949534
Test: atest DisplayManagerServiceTest.java
Change-Id: I1826dbd0cb7f4c4a76abbf25741b9862c5ff7355