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

(cherry picked from commit 84ddb34a)

Change-Id: I45e33865614d4c96f4a89cf117398666b556d500