simpleperf: improve cpu_hotplug_test.

1. Add offline_while_user_process_profiling test for
http://b/28086229.
2. Add options to adjust test duration and cpu hotplug
interval.
3. Report test time periodically for tests running for
a long time.

Bug: http://b/25193162
Bug: http://b/28086229
Bug: http://b/30839193

Change-Id: If02a45aed2dab718ca381b24da2898f22fed1a8d
1 file changed