Decouple Ipv6UpstreamRule with Ipv6DownstreamRule

Currently, Ipv6UpstreamRule is added follows the adding of
Ipv6DownstreamRule when it is the first rule from a downstream
interface to the upstream interface.

This CL changed it to:
1. Upstream interface change updates both IPv6 upstream and downstream
   forwarding rules.
2. Neighbor events updates only IPv6 downstream forwarding rules.

Data limit setting on the upstream interface is now depend on the
upstream rules instead of the downstream rules. This CL also updates
corresponding test cases in IpServerTest and BpfCoordinatorTest.

Test: atest TetheringTests
Bug: 261923493
Change-Id: I5b4df64a3d276afb7a24bbef3a0432f53377e8b2