Fixes animation pose breaking when dynamically changing between skeletal meshes that use the same animbp but have a different number of bones.
[CODEREVIEW] Laurent.Delayen, Lina.Halper
#ROBOMERGE-OWNER: ben.marsh
#ROBOMERGE-AUTHOR: aaron.cox
#ROBOMERGE-SOURCE: CL 6938273 via CL 6938276 via CL 6938281 via CL 6938283 via CL 6943622 via CL 6946612
#ROBOMERGE-BOT: BUILD (Main -> Dev-Build) (v367-6836689)
[CL 7089855 by aaron cox in Dev-Build branch]