[KV] Refactor clean-up

Only clean-up after possibly calling quota exceeded to avoid bug linked.
Small refactors and tests.

Test: atest KeyValueBackupTaskTest
Test: atest KeyValueQuotaTest
Bug: 114002519
Bug: 110082831
Change-Id: If53aaa59287cf9f2ac8d9ad47afc0a62161373c5