Remove reference to factory reset of K

The ISharedSecret.aidl description of the pre-shared secret K
mentioned that K was set up during factory reset.  While that could be
done, it is almost never done; most OEMs inject K in the factory.
Removed the reference to setup in factory reset to reduce confusion.

Test: N/A, comment-only change
Change-Id: Idbbd5a8d64197ef4713a75075a9730d06162ad05