PowerStats: adjust time since boot to epoch before writing to disk

Data stored to disk are extracted into an incident report for timeseries
visualization which shows walltime and may span multiple reboot sessions
so storing walltime is better suited than relative time since device boot.

Bug: 179712445
Tests: atest FrameworksServicesTests:PowerStatsServiceTest
Signed-off-by: Thierry Strudel <tstrudel@google.com>
Change-Id: I5ae8e2fe22f1ab9ea960e5be8fef4e9fb1cc10f0