init start time tracking.

With this change, init sets a property "init.start" to show the
CLOCK_BOOTTIME time at which init itself started, and for each service
an "init.svc.<name>.start" property to show the CLOCK_BOOTTIME time at
which that service was most recently started.

These times can be used by tools like bootstat to track boot time.

As part of this change, move init over to std::chrono. Also, rather than
make the command-line argument handling more complex, I've switched to
using an environment variable for communication between first- and
second-stage init, and added another environment variable to pass the
start time of the first stage through to the second stage.

Bug: http://b/32780225
Test: manual
Change-Id: Ia65a623e1866ea688b9a5433d6507926ce301dfe