Fix race condition in timer disarm/delete.

When setting a repeat timer using the SIGEV_THREAD mechanism, it's possible
that the callback can be called after the timer is disarmed or deleted.
This happens because the kernel can generate signals that the timer thread
will continue to handle even after the timer is supposed to be off.

Add two new tests to verify that disarming/deleting doesn't continue to
call the callback.

Modify the repeat test to finish more quickly than before.

Refactor the Counter implementation a bit.

Bug: 18039727
Change-Id: I73192c915cdacf608521b1792c54e5af14a34907