HommeDynamics: move timestamp update of PG fields out of fv_phys_dyn_to_fv_phys, and into initialize_impl

As noted by @ambrad in #2251 , the fix implemented in that PR introduced a redundant update of output fields, since `fv_phys_dyn_to_fv_phys` is called not only at init time, but also during the postprocess call at every time step. Although the double timestamp update is safe, it is redundant (and redundant code can generate confusion/bugs down the road), so it's best to move it out of that routine, and inside the calling function at init time.