Floss: use QuitWhenIdle for thread-safety

In |MessageLoopThread::ShutDown|, we invoke |QuitWhenIdleClosure| that
is not thread-safe, but |ShutDown| is meant to be called from a
different thread than the owner of the |MessageLoopThread|.

In this CL, we change it to the thread-safe variant, |QuitWhenIdle|.

Bug: 263808792
Tag: #floss
Test: Build and verify it doesn't crash when dis/enabling BT
Change-Id: Id2b8f7537b82b25a5950b35ecd615a554b89670e