Binding on-demand #1: TransportClient infra + PerformBackupTask usage

This CL introduces TransportClient infra-structure and integration in
TransportManager, plus a few unit tests. Also start using binding
on-demand for PerformBackupTask.

Ref: http://go/br-binding-on-demand
Bug: 17140907
Test: make RunFrameworksServicesRoboTests
Test: runtest -p com.android.server.backup frameworks-services
Test: adb shell bmgr backupnow <packages>
      With transports manually unbound and also canceled the operation a
      few times. Then inspected TransportClient logcat
Test: gts-tradefed run commandAndExit gts-dev -m GtsBackupTestCases
Test: gts-tradefed run commandAndExit gts-dev -m GtsBackupHostTestCases
Test: cts-tradefed run commandAndExit cts-dev -m CtsBackupTestCases
Test: Manually unbound before GTS/CTS above.
Change-Id: I63cbde27131205787d90663603a4f8f61d187607