dumpsys: fix formatting of service dump times

The current formatting of service dump times is
confusing some bugreport parsing tools. This is
because the current service dump time message
is hard to distinguish from the bugreport section
marker.

Adjust the formatting, to make the service
dump time message distinct from the bugreport
section marker.

BUG=30401031
TEST=manual

Manual test
$ adb bugreport /tmp/bugreport.zip
$ cd /tmp
$ mkdir br
$ mv bugreport.zip br
$ cd br
$ grep 'was the duration' bugreport*.txt
------ 4.283s was the duration of 'DUMPSYS MEMINFO' ------
------ 0.053s was the duration of 'DUMPSYS CPUINFO' ------
[...many lines omitted...]
--------- 0.003s was the duration of dumpsys AtCmdFwd
--------- 0.002s was the duration of dumpsys DockObserver
[...many more lines omitted...]

Change-Id: I9d3dd75210b3984240a4b7ab49a1cc6124a269d8
(cherry picked from commit 50b7e039)