Separately report thread suspension and garbage collection times.

Separately report thread suspension and garbage collection times.

The GC elapsed time written to the log used to be the sum of the time
spent suspending threads and the time spent executing the rest of the
garbage collector code.  With this change we account for each of these
phases separately.  With the concurrent GC, thread suspension can
dominate the GC pause times so breaking this number out gives a clear
picture of the efficiency of the underlying GC algorithm.

Also, accounting for the concurrent garbage collector is more
accurate.  The start and end times did not subtract the amount of time
spent during the concurrent mark.  The clock is now stopped and
restarted during the concurrent phase of the collector.

Change-Id: Icf3f8c0e183166a62ca1bf32fc77f17e3b5aa963