Fix deadlock in timer_delete.

If the callback function for a timer did a timer_delete, the function
would never return. The problem was that the timer_delete function would try
to wait until the timer thread has finished. Waiting for yourself to finish
doesn't work very well.

Bug: 13397340
Change-Id: Ica123a5bafbc8660c8a4a909e5c2dead55ca429d