debuggerd: properly unblock signal handler.

This change ensures that debuggered properly releases
the signal handler that invoked it after the PTRACE_ATTACH.

The previous code simply did a close() of the file descriptor,
but for some reason, this didn't always make the read() blocking
the signal handler exit. Instead, the thread would stay blocked
and never fault, preventing the generation of a useful stack
trace.

Change-Id: I6b0579041165a710d74ec1bece113ff7b828aed4