Randomize the prefix used by RouterAdvertisementResponder. am: 6ab502437e

Original change: https://android-review.googlesource.com/c/platform/frameworks/libs/net/+/2173507

Change-Id: Iae3ccaf7156f3541b8804258d5321fe4f504da4a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>