[Remote functions] Only set the default runner *after* resolving the remote FLR.

Previously, if the `runner` was not specified for a function
execution, we would immediately set it to the default runner of the
*local* FLR, even if the function was to be executed remotely. This
change postpones the resolution of the default runner until after the
function invocation has been routed to the FLR that will actually
execute it.

As a result, we avoid the pathological case where a GPU device using a
private threadpool (TF_GPU_THREAD_MODE=gpu_private) ends up running
all of the ops for the CPU-side input pipeline on the private
threadpool.

PiperOrigin-RevId: 195735734