Fix how the ProfileSaver thread attaches to the runtime.

If the runtime is shutting down we can't attach new threads. In this
case it is expected that Runtime::Attach returns nullptr.

Also, lower the serverity of the log to warning so that it does not
show up in the run-tests.

Bug: 28435649

(cherry picked from commit e55fda1373abad5ace4409453c51aeb0daaf99ef)

Change-Id: I96464d76a275d155a90a1bc3bb99fbb9d914cf20
diff --git a/runtime/thread.cc b/runtime/thread.cc
index e3feda6..fb24828 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -740,7 +740,7 @@
   {
     MutexLock mu(nullptr, *Locks::runtime_shutdown_lock_);
     if (runtime->IsShuttingDownLocked()) {
-      LOG(ERROR) << "Thread attaching while runtime is shutting down: " << thread_name;
+      LOG(WARNING) << "Thread attaching while runtime is shutting down: " << thread_name;
       return nullptr;
     } else {
       Runtime::Current()->StartThreadBirth();