Add tests for NATT keepalives stopped internally in KeepaliveTracker. am: 3d901553a6

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/23667822

Change-Id: Iabe47ea1948ac00ce2488b93bee565f117b17188
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>