NR UE: correctly apply Doppler and TA for NTN target cell

Make sure to apply Doppler and TA not directly when receiving the target cell ntn_config,
but apply Doppler just before the initial sync, and the TA after the initial sync when
we have tha actuel cell frame and slot numbers.

nfapi/open-nFAPI/nfapi/public_inc/fapi_nr_ue_interface.h

@@ -588,6 +588,7 @@ typedef struct {
   // cell scheduling offset expressed in terms of 15kHz SCS
   long cell_specific_k_offset;
 
+  bool is_targetcell;
   bool params_changed;
 } fapi_nr_ntn_config_t;