Fix PreInit not being called on XR plugins when the -hmd parameter is specified
#jira UE-70061
#rb Nick.Whiting
#lockdown cristina.riveron
#ROBOMERGE-OWNER: lina.halper
#ROBOMERGE-AUTHOR: jules.blok
#ROBOMERGE-SOURCE: CL 5932461 in //UE4/Release-4.22/... via CL 5933171
#ROBOMERGE-BOT: ANIM (Main -> Dev-Anim)
[CL 5945756 by jules blok in Dev-Anim branch]