Fix debuggerd issues.

- Fix a problem where a tid exits before the attach completes, and it
  causes debuggerd to self terminate.
- Fix a problem where sibling tid dumps do not properly wait for the tid
  to get signalled.

Bug: 17800180
Bug: 12567315
Change-Id: Ic3cd619cc2c72402f9a45f14abeed4721b50d64d