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

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

Change-Id: I90864e1c4693bc30e4dcb5e6e1b39a8e933f3591
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>