Add and use OstreeAsyncProgressFinish helper

This is a g_autoptr version of OstreeAsyncProgress that also
calls ostree_async_progress_finish() before being freed.

This should be used in all "leaf" functions that creates an asyncprogress
to avoid leaking any idle change idle sources. Using a auto* means
some code can be cleaned up to avoid goto out style handling for this.

Also, this adds a missing finish() in
_flatpak_dir_fetch_remote_state_metadata_branch().
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