Add util method to generate IPv4-mapped IPv6 address from IPv4 address Cherry-pick of aosp/2795709 to backport VPN security fix to non-mainline devices. Having Merged-In to prevent automerger to udc-mainlie-prod where frameworks/libs/net does not exist. Bug: 193031925 Test: TH (cherry picked from https://googleplex-android-review.googlesource.com/q/commit:03eb6f09360de988687c06940b6f13be7e650b34) (cherry picked from https://googleplex-android-review.googlesource.com/q/commit:29e53bd862d7b4c30552f188065e1239e84c401e) Merged-In: Iaa5d5f31904f8a122cb96f2c3f0e3499a2577664 Change-Id: Iaa5d5f31904f8a122cb96f2c3f0e3499a2577664