Fixed crash when RAT changes with tethering on

We should use the orignal APN setting used for data setup
to determine if the data connection has DUN capability.

Test: Manual
Bug: 78592192
Merged-In: I57c823061d49238c9c76deb54c59dbcf95153108
Change-Id: I57c823061d49238c9c76deb54c59dbcf95153108
(cherry picked from commit 3ced483c)