DevicePolicyManager#removeKeyPair- update contract

May also return true if the keypair didn't exist in the first place
which is a small reduction in the amount of information leakage, but
more importantly lets a caller be sure about keystore state at the end
of their call.

Bug: 27335182
Change-Id: I5e6d4c599b74f031e3f6d17cb7540898ffaf2571