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
2 files changed