Switch "UTC time" to "Unix epoch time"

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: I4c5435bdb3bb124d4cb465a966bd43d58b640421