Increase PerformBackupTask unit coverage 3

With KV Refactor in mind:
* Tests around calling agent.
* Tests around agent writing data.
* Tests around transport handling agent data.
* Small modification on ShadowBackupData{Input,Output} to allow empty data.
* Small refactor.

Test: atest PerformBackupTaskTest
Change-Id: Ia9296859926d09d5bbcb0532fb869ace110240e4