Fix race condition on cancelling alarms

There was a race condition in which pulling alarms can be cancellled
after they are initialized, causing bucket boundary pulls to fail.

Test: manually verifified alarm consistently cancelled before being set
Test: manually verified alarms were properly cancelled on statsd crash
Fix: 156855909
Change-Id: I913b6f3e3d03269fb9283199f8b9d666a298e145