init: ro.boottime.init.first_stage

Add a property ro.boottime.init.first_stage to provide us a
first stage init duration from start to exec completed in
nanoseconds.

For consistency, report nanoseconds duration for
ro.boottime.init.selinux as well instead of milliseconds.
Now also report consistently from start to exec completed
instead of just the selinux load time.

SideEffects: ro.boottime.init.selinux is reported to TRON and
             may alarm with the millionfold increase in precision.
             ro.boottime.init is now also consistent with ns
             precision.

Test: inspect
Bug: 124491153
Bug: 129780532
Change-Id: Iff4f1a3a1ab7ff0a309c278724c92da0832b9a69