[TNU06] Add roaming notification

Warn user of potential data charges if the backhaul is
cellular and user is roaming.

Bug: 145629001
Test: atest TetheringTests
Change-Id: I74b4f87c2f6aad09e05d3f2a779f880396885953
Merged-In: I74b4f87c2f6aad09e05d3f2a779f880396885953
(cherry picked from commit 1af69e5b, aosp/1237026)