Please support explicit proxy settings on Android

### Describe the feature you'd like to request

The floccus browser extension uses the browser's proxy settings - at least in Firefox, which is what I've tested so far. However, there is no corresponding way to do this on Android.

In particular, I'm interested in accessing my own server which is a Tor Onion (hidden) service, but only for specific applications, without pushing all my traffic through Tor via a system-wide VPN or system proxy settings. This is supported by other private backup tools such as DAVx5 and Syncthing.

### Describe the solution you'd like

The ability to use a local SOCKS proxy for floccus to access the bookmarks server. This could either be for the whole application or per account; the former is simpler and sufficient for my use-case, and it's what DAVx5 and Syncthing both do.

### Describe alternatives you've considered

A workaround is to enable the system-wide VPN on Orbot, but it's very inconvenient to have to switch it on and off every time I want to sync my bookmarks, and it prevents non-Tor VPNs from working at the same time.