Add test for tetherOffloadClient{Add, Remove, Clear} am: dad664748a

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/1744606

Change-Id: Ifb777d6cccdd15ee3a5c0af81c9541d7c54ebdc6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>