Correct the description for getKeyCharacteristics

The description should note that keystore-enforced tags are not to be
returned. This is done so that the keymint implementation doesn't have
to bother keeping track of tags it's not repsonsible for dealing with.

Bug: 186685601
Test: none (it's just a comment change)
Change-Id: I3ff94201c262a5071d271b150dbbf21888d678aa