Keymaster key validity dates are optional.

This CL makes Android Keystore framework code add
KM_TAG_ACTIVE_DATETIME, KM_TAG_ORIGINATION_EXPIRE_DATETIME, and
KM_TAG_USAGE_EXPIRE_DATETIME tags to the authorizations set only
if the corresponding time instants were specified through the
framework-level API. This is fine because these tags are optional as
it turns out.

Bug: 18088752
Change-Id: I6a5ae4cadb441e61576231815e6bec6e9248bc72