Set DF flag on NAT-T keepalive packet am: c6a2f6faf5 am: 0427777fe0 am: e8420f465b

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/2616795

Change-Id: I6864c52c1f5bc641cc6b022679634e1d981d9776
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>