Map realtime to clock_monotonic.

This maps a monotonic timestamp to the
corresponding real-time timestamp, which
can be used to match up the traces with
other logs that use real-time.

Also write clock_sync records first instead of at
the end, to avoid not being to write it due to the
buffer being full.

Bug: 23668823
Change-Id: I644aeea496197e194ec30f808f754e3e043d905f