Control threads do not respect device tracker options

If a program calls vtkm::cont::Initialize and selects a particular device to use (i.e. through command line arguments), those settings only take effect on the main thread. Any other spawned thread gets its own thread-local RuntimeDeviceTracker with default options.

When a new local RuntimeDeviceTracker is created, it should grab the state of the RuntimeDeviceTracker on the "main" (initialized) thread to get these defaults.