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

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

Change-Id: If2c26d605b2b0438fdb19d7196a0d2eff00de73c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>