The amplitude-js library gained React Native support so there is no need to keep
separate implementations.

The previous one no llonger works out of the box with RN.

Ever since https://github.com/facebook/react-native/pull/23674 landed it has
been possible to run timers in the background, assuming your app is allowed to
run in the background already, as is our case. So, stop using the library on
iOS, which will avoid creatring needless backgound tasks.

Move all polyfills to a standalone feature, which gets imported before anything
else in the mobile entrypoint. This guarantees that any further import sees the
polyfilled environment.