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

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

Change-Id: I15950033cd3d2c8f2830d1a089b8cd395c087e7f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>