[TNU1.1] Update tethering notification strings

Update string for no upstream and cellular roaming notification.

Bug: 145629001
Bug: 147818698
Test: atest TetheringTests
Change-Id: I7302cf351b0a9a42ad5a543f61ab084de189f282
Merged-In: I7302cf351b0a9a42ad5a543f61ab084de189f282
(cherry picked from aosp/1261866)