Rename "PacketTransportInterface" to "PacketTransportInternal".

This is the naming scheme we've been using for internal interfaces.

Also, this CL will introduce a PacketTransportInterface in the webrtc namespace,
which would get too easily confused with the rtc:: one:
https://codereview.webrtc.org/2675173003/

BUG=None

Review-Url: https://codereview.webrtc.org/2679103006
Cr-Original-Commit-Position: refs/heads/master@{#16539}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5bd5ca344e04106c41c3f3d5c7195db62a5e63a1