Use Keymaster-friendly validity dates.

Keymaster HAL currently requires that key validity start and end dates
always be specified. The framework API does not. This CL expresses
the framework API's "not specified" instants to Keymaster as instants
in distant past or future.

Bug: 18088752
Change-Id: Ia9d66d5e57bfca30628cdef6e0925a2781a3acfb