wifi: Add RTT structure conversion methods

While there,
Hide |convertLegacyIeBlobToHidl| from hidl_struct_util.h. There is no
need for any of the HIDL objects to directly use this. They will be used
internally in |convertLegacyScanResultToHidl|.

Bug: 31991232
Test: Compiles
Change-Id: I5702906ea84e3c77fece7be5f9d48ff3ae418c41