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: 242928495
Change-Id: Ie2bdd2151dae26a611c3905342bb49ddddf2788d
Merged-In: Ie2bdd2151dae26a611c3905342bb49ddddf2788d
(cherry picked from commit e6073bc3)