refactor: optimize plugins for angular treeshaking

BREAKING CHANGE: You can remove the plugin import from the app.module.ts because of the changed Inject decorator

src/@ionic-native/plugins/firebase-crashlytics/index.ts

@@ -30,7 +30,9 @@ import { Cordova, IonicNativePlugin, Plugin } from '@ionic-native/core';
   installVariables: ['ANDROID_FIREBASE_CORE_VERSION'],
   platforms: ['Android', 'iOS']
 })
-@Injectable()
+@Injectable({
+  providedIn: 'root'
+})
 export class FirebaseCrashlytics extends IonicNativePlugin {
   /**
    * Simply add the plugin to get the default Crashlytics functionality. Note that crashes and logged exceptions will only be reported when the application restarts. In order to log caught exceptions the following can be used: