[KV] Exceptions for error-handling

Added 2 exceptions:
* TaskException: For what we used to call transport-level failures,
  these bring queue processing to a halt.
* AgentException: For failures that happen due to the backup agent, these
  only prevent the backup of the current package, still allowing backup
  for the remaining packages in the queue.

These are usually thrown deep in the call stack and caught in backupPackage()
(and similarly backupPm()) and run(), the former for clean-up, where
they are re-thrown and the latter for further processing where they are
finally swallowed. The clean-up is more explicit now.

This enabled further refactoring of backupPackage()/backupPm(),
extractAgentData() and sendDataToTransport().

One change that I intend to revisit is reporting to the observer.
Previously we used to detect some exceptional cases and set mStatus
(which doesn't exist anymore) to other more general exceptional case and
then in this general case handling report the success/failure to the observer.
With the new exception-throwing model I changed this and the leaves are
actually responsible for reporting success/failure to the observer (see
changes in the reporter). This is to avoid too many changes in this CL.

I'm entertaining the idea of extracting out package backup as a separate
class and leave only queue-processing and generic bookkeeping to this
class.

Test: atest FrameworksServicesRoboTests
Test: adb shell bmgr backupnow <kv_package>
Change-Id: If3b1da7a5eb939e453f94fad76f5790696d1265a