Rename TransportClient to TransportConnection

The change is auto-generated through IDE rename function and makes the
following naming changes:

1. TransportClient -> TransportConnection
2. TransportClientManager -> TransportConnectionManager
3. + corresponding test files

TransportConnection is a more appropriate name for the class as it's
exactly what it does - manages the connection to the remote
BackupTransport service implementation.

This is a preparatory change to making the BackupTransport AIDL async.
TransportClient name will later be used more appropriately for a class
that wraps the actual communication with the transport.

Bug: 202716271
Test: m -j
Change-Id: I76a98edc7102c8fcffdb050208e9e65543e6e10c