Federated implementations

[trik]

Following the discussion in #544, i'm trying to publish a web implementation of this plugin but i'm facing a couple of issues:

• ReactiveBlePlatform class exposes the set instance method to enable other platform-specific implementations but the instance is currently always re-set using ReactiveBleMobilePlatformFactory https://github.com/PhilipsHue/flutter_reactive_ble/blob/master/packages/flutter_reactive_ble/lib/src/reactive_ble.dart#L107

• platformIsAndroid variable is currently set using Platform.isAndroid, which rises an exeception in non-native environments

[trik]

@Taym95 i think this code needs to be refactored. it's working good with a custom implementation but it's throwing a LateInitializationError when using the default one (reactive_ble_mobile).

try {
  ReactiveBlePlatform.instance;
} on LateInitializationError {
  ReactiveBlePlatform.instance = const ReactiveBleMobilePlatformFactory().create();
}