vold: Do not cache CE keys in vold

CE keys were cached in vold to support untrusted reset
by a device admin, this is now supported by Locksettingservice
using synthetic password. This change requires a secret to be
provided to retrieve the CE key and re-wrap without the secret
when user removes the credential.

Test: Set credential, remove credential, swipe to none
and vice-versa.
Bug: 26948053
Change-Id: I4cb1c035a472477e70c1ff5bf0b2c3fcfad495e5