Background import operations should wait for existing import operations to finish

[vin047]

Describe the bug
In background mode, if there are any existing import operations in progress, the background import operation will run simultaneously with those existing operations. This can result in assets being processed for import (including termination) twice. Note that this shouldn't produce duplicated data – asset should be skipped during individual operations, overwritten, or in case of a termination, attempts to terminate again.

To Reproduce

1. Enable Debug logs and Backup & Sync.
2. Leave phone on standby and on charge to begin background process.
3. Later, observe in the logs that, once the app resumes in background mode, existing imports continue to process whilst background import begins to process the same assets for import.

Expected behavior
Background imports should not process assets that are in progress already.

Additional context
App enters suspended state when user leaves app. Later, on entry to background state, if app wasn't deallocated from memory, imports resume from where they left off. Background operations start and request unimported assets from the local database, which includes assets that are still being processed by existing imports.

Dependent on #16