Cleaning up duplicate proto for ProcessStartTime

There is no need to maintain two copies of the same proto message.
Moving framework to directly use the canonical proto used by statsd.

Test: m -j statsd_testdrive
out/host/linux-x86/bin/statsd_testdrive 169

Bug: 241474647
Change-Id: Ie2bdd2151dae26a611c3905342bb49ddddf2788d