debuggerd: fix debuggerd -b <pid>

Dump all of the threads when using `debuggerd -b`.

Bug: http://b/34719257
Test: `debuggerd -b mediaserver`
Change-Id: I2f57100985e5b0c2bf019482ba9feff31921692f

debuggerd/crash_dump.cpp

@@ -322,7 +322,7 @@ int main(int argc, char** argv) {
   bool fatal_signal = signo != DEBUGGER_SIGNAL;
   std::set<pid_t> siblings;
   std::set<pid_t> attached_siblings;
-  if (fatal_signal) {
+  if (fatal_signal || backtrace) {
     if (!android::procinfo::GetProcessTids(target, &siblings)) {
       PLOG(FATAL) << "failed to get process siblings";
     }