Add java class for Ingress discard bpf map key value

Cherry-pick of aosp/2795708 to backport VPN security fix to non-mainline
U devices.

Bug: 193031925
Test: TH
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:9024892db8de73eea86967880b0cfa4470db079f)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:a25019c5917a6b1a5070bff2d897963cbecb1b5d)
Merged-In: Ib403099df5cefa3cbad52e3fd568b1a5047adcd4
Change-Id: Ib403099df5cefa3cbad52e3fd568b1a5047adcd4