[web/native/lib] Combine openSocketSelector from web and native

Summary:
ENG-6043

Should have been done when web cookies were migrated to redux, but I haven't noticed this code then. Now that cookies are in redux on all platforms we can merge these two functions.

Test Plan: Tested that web app still works fine (logging in, out, navigation, sending messages). It's hard to test the of cookie change restarts the websocket because web can't really recover from cookies changes and it just causes a logout.

Reviewers: inka, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10309