[KV] State machine to linear task w/ RemoteCall

This is the first CL of key-value backup refactor.

* Method execute() that executed the state machine states is now empty
  and run() is created with the initial version of the linear task. It's
  basically composed of begin + loop + end. Although it still has the notion
  of state, it's more restricted than before (with private methods returning
  the next state). This is intentional to avoid making this CL too heavy to
  review and to avoid too much behavioral changes here. In the next CLs I
  intend to remove BackupState.

* Introduction of RemoteCall, which encapsulates an outbound call that
  leaves the system_server, with time-out and cancellation built-in.
  Agent calls are now triggered using this. As a result there is no more
  operationComplete() method either.

* Cancellation now is cleaner. We don't need a step lock anymore, only a
  (volatile) boolean that is checked in every queue iteration. If asked
  to cancel during an ongoing agent call we rely on RemoteCall.cancel()
  to return control of the task thread to us. We wait for the cancel
  acknowledgement to preserve the contract of no more transport calls.

* PFTBT instantiation moved from the constructor to the run() method,
  which makes more sense.

* No need for mFinished, mBackupData == null bookkeeping since time-outs,
  cancellation and legitimate agent responses won't step into one another
  anymore.

* Ternary (mQueue.isEmpty) ? BackupState.FINAL : BackupState.RUNNING_QUEUE gone
  because we check this in the beginning of invokeNextAgent() and now we
  don't pay the state-machine tick price associated with the handler.

* PerformBackupTask call sites now call static method start(), that
  spins up a new dedicated thread for the task. This new thread is
  assigned the same (process) priority as the backup thread
  (THREAD_PRIORITY_BACKGROUND).

Work left for future CLs:

* RemoteCall spins up a new thread for kicking off the call, this is for
  system agents that are executed inline. Old PBT also executed in the same
  thread (backup handler thread), so maintaining this to keep this CL at
  a reasonable size.

Test: atest PerformBackupTaskTest
Test: atest RunFrameworksServicesRoboTests
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/backup
Test: atest CtsBackupTestCases
Test: atest CtsBackupHostTestCases
Test: atest GtsBackupTestCases
Test: atest GtsBackupHostTestCases
Test: adb shell bmgr backupnow <kv_packages>
Test: 1. adb shell bmgr backup <p1> <p2>
      2. adb shell bmgr run
      A) 3. Cancel while sending <p1> data to transport
         4. Verify <p1> is backed-up and not pending
	    Verify <p2> is not backed-up and is pending
      B) 3. Cancel while waiting for <p1> agent
         4. Verify <p1> is not backed-up and is pending
	    Verify <p2> is not backed-up and is pending

Change-Id: Ia456c4e807de95d662c9e923245be07e8996f58a