Fix concurrent modification crash in onAlarm

Change-Id: Idfd094f3c9ea59356440d6851ccd5abda36ca6ba
Fixes: 35640585
Test: manual; after boot, opened the power menu to force
system_server to spin up a ThreadedRenderer instance
(and thus register itself as a callback on GraphicsStatsService).
Then manually set the date forward by a day to trigger onAlarm
and verified the system didn't reboot/crash. A systrace capture
verified that the alarm fired and package:android (system_server)
had a log rotation event.