Fix for bug in Fix for wrongly initialized test key

With the last patch I exposed the auth token key to users of the
GenericOperation. But because the auth token key is never used in the
default implementation it did not show that the setHmacKey method did
not actually work. This patch fixes it.

Bug: 78456249
Test: Compile trusted app based on the GenericOperation
Change-Id: I25a80f985ce500e3658421cac62421133cc5b788