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
Change-Id: I208bcd6f0f7e35ae6471e263dee00d3ab68b3b8f
2 files changed