MessageLoopThread: Release lock during blocked StartUp/ShutDown

During StartUp/ShutDown(), if another running task in message loop needs
api_lock, then it will deadlock. Release api_lock in StartUp and
ShutDown when current thread is blocked by another task

Bug: 116081383
Test: run unit test
Change-Id: Ic34406d0cfc6364ac965c4f1aa917a5ccaf6b373