Add downloads management support to DownloadManager

[keianhzo]

In FxR we are currently using a custom DownloadManager based implementation for Downloads. We would like to migrate to FetchDownloadManager to unify the Http stack using a GeckoWebExecutor.

One of the issues we have found is that FetchDownloadManager doesn't support Downloads querying/removing and we need that to update the UI: get downloads status, remove a download/s, etc

It would help if FetchDownloadManager provided a querying mechanism similar to the DownloadManager query and remove methods so we can get the current downloads status or cancel/stop/remove downloads.

As an alternative the completed downloads could be fed to the DownloadManager so we could always get the finished downloads and get the ongoing through onDownloadStopped but we would still be missing some downloads management methods like cancel/stop/remove.

┆Issue is synchronized with this Jira Task

[pocmo]

For the in-memory state (e.g. showing status) #7050 will be helpful since that will move all the internal state from the service to our browser store where components the app (and UI) can use it. (which may be another reason to use the FxR <-> AC bridge). With that I think we may also be able to provide "use cases" that allow to cancel, stop etc. currently active downloads. Adding a persistent storage to that is something we eventually want to do too, but that is a bigger project.