Move transport client code into its own lib

We will route backup through the BackupEncryption APK which will
implement an intermediate transport which will encrypt (or decrypt) the data when
sending it (receiving it) from the real transport.

Since both backup services and IntermediateTransport need to bind to
IBackupTransport, they both need the transport client code so we move that
into its own lib.

Bug: 139269919

Test: atest -v RunBackupFrameworksServicesRoboTests
Test: atest -v $(find frameworks/base/services/tests/servicestests/src/com/android/server/backup -name '\''*Test.java'\'')'
Test: atest -v CtsBackupTestCases CtsBackupHostTestCases
Change-Id: Id986162ac71adf8638e5797169ef70e0d3d58b0c