DataLoader lifecycle.

- restarting DataLoaders for not fully downloaded mounts,
- exponential backoff retry on DataLoader restart.

Bug: 173223115
Bug: 160634487
Test: atest PackageManagerShellCommandTest PackageManagerShellCommandIncrementalTest IncrementalServiceTest PackageManagerServiceTest ChecksumsTest
Change-Id: I5a93fd515ef0fec09452ce7bb4cd79fbd283d45f