Fetch RSS feeds via Tor
Use the localhost Socks proxy to fetch the feeds. The proxy port is currently set in briar-android and would need to be moved to core or api.
The FeedManager should listen to TransportEnabledEvents and enable the feed fetching scheduler when Tor is started first. For simplicity, it should not turn off the scheduler when Tor gets turned off, but let the fetch job fail cleanly when Tor is not running.
We should use stream isolation to ensure the lookups for different feeds happen over different circuits. This is done by specifying a different SOCKS username and password for each feed (any username and password can be used, they just need to be different - generating them on the fly would be fine).