Trust user-added CAs on Android

[gnprice]

This is a configuration change that was requested a few times over the years for zulip-mobile, as zulip/zulip-mobile#3312, before we got a contribution implementing it as zulip/zulip-mobile#5493 . We should do the same thing here.

The implementation for zulip-mobile (see zulip/zulip-mobile@85c3a71) is entirely at the Android metadata layer, so we should be able to pretty much just copy-paste that.

[gnprice]

This feature is in the new beta release v0.0.9. @shrizza please give it a try — I'd be glad to have confirmation of whether it works for you.

[gnprice]

Seems like that version doesn't yet work — but there is hope that a small further tweak to the manifest will make it work. Details in chat:
https://chat.zulip.org/#narrow/stream/48-mobile/topic/flutter.3A.20user-added.20certs/near/1716835