Better solution for per-platform support

[brodycj]

The existing solution needs one JavaScript file per platform, as described in lelandrichardson/react-primitives#136 (comment).

I think a better solution would be to check for absence vs presence of non-React Native navigator variable as suggested in necolas/react-native-web#1146 (comment).

I think this idea should both alleviate per-platform maintenance need and avoid some potentially risky assumptions about the build tooling used.