selectBackupTransportAsync should report failure when backup is disabled

Fix: 37304539
Test: runtest -x frameworks/base/services/tests/servicestests/src/com/android/server/backup/TrampolineTest.java
Test: setup device owner, then set up profile.
      Ensure tapping Settings->Google->Work->Security -> Verify app
      is working

Change-Id: I393308857ea6082dca167e3e417b946615a83c20