Move transport client code back to the main target

In ag/9433204 this code was separated into its own library for a feature
that was cancelled. It doesn't make sense to keep it separate anymore.

Test: m
  atest BackupTransportClientTest.java
  atest CtsBackupHostTestCases
Bug: 210083465
Change-Id: I6b5a6bb45563c16b359f0281aac2c28bce03a6fd