init: fix process restarting

The time data types associated with restarting processes halfway moved
to std::chrono and halfway didn't.  In this intermediate state, the
times would get converted from nanoseconds to seconds then to
milliseconds.  The precision lost when converting to seconds would
cause the main loop of init to spin whenever a process was within a
second of being restarted.

This patch cleans up this logic and uses nanoseconds and milliseconds
explicitly, with a ceiling to milliseconds to prevent unneeded
spinning.

Test: boot bullhead, kill processes, see that they restart sanely.

Change-Id: I0b017ba0e50c09704b0c5cdfcde1dba461804593