libpdx_uds: Always create channel sockets in the server process

Whenever a client connects to PDX service endpoint, the connected
socket was being used as the channel connection. This makes it
difficult to ensure that the socket has the appropriate SELinux
label applied.

Changed the implementation to make sure that the actual channel socket
pair is created in one place in the server process.

The new Endpoint::CreateChannelSocketPair() is now being used to
create the channel sockets when an incoming connection is accepted
and when PushChannel request is received.

The newly created channel socket is sent to the client in response
to connection request.

Bug: 37646189
Test: Device boots. CubeSea launches.
Change-Id: Id3158484aeb18007f18b279afc60613076f80f14