fix(rn-expo): fixes error/warning when the call is left in expo [skip ci]

[kristian-mkd]

Overview

Fixes the error/warning generated when user is leaving a call in our sample expo app.

 ERROR  Warning: Cannot update a component (ForwardRef(BaseNavigationContainer)) while rendering a different component (JoinMeetingScreen). To locate the bad setState() call inside JoinMeetingScreen, follow the stack trace as described in https://reactjs.org/link/setstate-in-render

The issue was in our JoinMeetingScreen component - it's trying to update state during render.

Solution: Move the navigation logic into an effect: