Pick default interface for upstream.

With multiple upstream interfaces active and connected,
we should choose the default interface for upstream.
CRs-fixed: 461266

Change-Id: Ia3bc32300a1f77f46237994d0e38e9c9886f4bea
frameworks/base: IPV6 tethering support

This patch adds two new API calls addUpstreamV6Interface and
removeUpstreamV6Interface to the tethering framework to pass the IPv6
upstream and downstream interfaces to netd if IPv6 is available.
Required to enable ICMPv6 forwarding.

Change-Id: I1f4f6284b2f152d8b4f0fbd02ac5a17f57c71dd7