Add ipconnectivity.proto to services jar

This patch moves ipconnectivity.proto and companion serialization
classes from /vendor/google/app/ConnectivityMetrics into the framework
services jar.

This will allow to implement a dumpsys interface for IpConnectivity
metrics events defined in android.net.metrics that output metrics as
a serialized proto.

Bug: 31254800
Change-Id: Ia4449103fd7222c9d899352fea2dce47a790a49c