More sensible return code for Credentials.deleteAll

Was: result = True iff nDeleted != 0
Now: result = True iff nDeleted == nExisted

The most common reason you'd want to delete all credentials under an
alias is to be sure they no longer exist. The new contract gives a
way to do this without multiple IPCs to the same service.

Bug: 27335182
Change-Id: I8762b9b4fcc48037387dd805dbd0dbbe141d5b24