Move Connection off of RefBase

The Connection class doesn't need to be inside sp<>. Convert it to use
shared_ptr. There are still a couple of places where the connection is
passed around, so we can't make it a unique_ptr.

Bug: 211379801
Test: m checkinput
Change-Id: I6755e7c9a39a015d30738c1b9052733c182628d5