[TNU1.1]Add tethering notification strings

Add string for no upstream and cellular roaming notification.

Bug: 145629001
Bug: 147818698
Test: atest TetheringTests
Change-Id: I30f68d83344f66fb3ef77abf3f8748c3eb1276f0
Merged-In: I30f68d83344f66fb3ef77abf3f8748c3eb1276f0
(cherry picked from aosp/1260112)