signal_catcher: Only notify tombstoned when we're using it.

This would otherwise cause a benign warning message when tombstoned
based stack dumping was turned off. The notify would fail immediately
with an EPERM due to SELinux restrictions.

Bug: 62779030
Test: test-art-host
Change-Id: Icc92cf0ebf146091317bb48b9968e20e9ab99da5
diff --git a/runtime/signal_catcher.cc b/runtime/signal_catcher.cc
index 8c934d5..f0b6ee4 100644
--- a/runtime/signal_catcher.cc
+++ b/runtime/signal_catcher.cc
@@ -168,7 +168,7 @@
   }
 
 #if defined(ART_TARGET_ANDROID)
-  if (!tombstoned_notify_completion(tombstone_fd)) {
+  if (use_tombstoned_stack_trace_fd_ && !tombstoned_notify_completion(tombstone_fd)) {
     LOG(WARNING) << "Unable to notify tombstoned of dump completion.";
   }
 #endif