Move load_user_creds out of StackWatch

This method was moved out of EngineService in a recent refactor
however load_user_creds not related to stack watching, so this
method is better off living in EngineService.

Change-Id: I67fa06df5c24baa901dc6314205ea09d012437fb
Merge "Move load_user_creds out of StackWatch"