Add test for tetherOffloadClient{Add, Remove, Clear} am: dad664748a am: 32244e9034 am: 7c962de85f

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

Change-Id: Ie2fe3c138898362b4d0a0d7a5935b52d2d032f73
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>