This commit makes creating new RestoreSets explicit.
Initializing a backup transport now actually cleans its data as the AOSP
documentation demands. This should be fine as we usually do a fresh
backup after a new initialization.
Contrary to before, an initialization does not create new RestoreSets
anymore, but works within the existing set. For now, only manually
choosing a new storage location creates a new RestoreSet.
Loading cursors can happen with cloud-based documents providers
such as Nextcloud.
When they return a cursor that is still loading,
we might continue with stale information.
So now we wait for a loading cursor to be fully loaded
before continuing.
