bpf tethering offload - add src subnet to upstream ipv6 direction

This patch is based on aosp/2535559 from maze@.

Add source prefix into the upstream key such that only packets which
source IPv6 address matches it will be forwarded to the upstream
interface.

In this patch, the source prefix is set to zero so there is no
behavior changes. Next CL in patch series will use the real source
prefixes retrieved from upstream interface.

Test: atest TetheringTests
Bug: 261923493
Change-Id: I43d068a29b937c7dfeb6fab632a8effb47ee2263