Fix up upstream interface types for case DUN_UNSPECIFIED

The existing logic will add TYPE_MOBILE and TYPE_MOBILE_HIPRI
even if the carrier config has TYPE_DUN for DUN_UNSPECIFIED.

A check is added not to modify the upstream interface types
if there is already a cellular interface type. Add TYPE_MOBILE
and TYPE_MOBILE_HIPRI if there is no cellular types found.

Test: Hotspot on Verizon and T-Mobile US.
      Verified the requestNetwork has the DUN capability.

Bug: 38186915
Change-Id: I74fc5c791fee2885bf66df8975e92c2b48f1668c