Switch "UTC time" to "Unix epoch time" (core)

This is more correct. Android devices, even under ideal conditions,
don't track UTC (which contains leap seconds and uses the SI definition
of a second) but use a clock based on Unix epoch time (no leap seconds,
but clocks may skip/stop or smear around leap seconds to ensure a fixed
number of second increments per calendar day when leap seconds are
applied to UTC).

Bug: 218802673
Test: Compile only
Change-Id: I0c557e2231f87ea1b99e3935f79af15bccf91da4