Add getters to UnderlyingNetworkInfo

Address API review feedback, add getters to UnderlyingNetworkInfo
instead of exposing fields.

Instead of wasting memory by converting this into an array, have
migrateTun take a List<String>. In turn, tunAdjustmentInit should
also take a List<String>.

(cherry picked from ag/14211075)
Bug: 183972554
Test: atest android.net.UnderlyingNetworkInfoTest
Merged-In: Id59744097208d91298a25ef110ade91a9cf291a1
Change-Id: Id59744097208d91298a25ef110ade91a9cf291a1