Only one crashing thread should contact debuggerd.

If two or more threads crash at the same time, only let one talk to
debuggerd. It's possible for a race to occur that two threads send
data to debuggerd, the second one will cause errors in debuggerd since
the process will die once debuggerd lets the crashing pid start again.

Bug: 19183955
Change-Id: I17dfce46102117ab4a870f7381bd526488d37fb5