* Fix de-selecting exchange providers not getting saved
* only change field value when the amount is better than the already existing one
* Show calculated amount after getting the best value from all providers
* Catch exceptions to avoid stopping the providers calculate amount APIs
* Fix Splay map only saving the last value obtained
* Show limits for the provider with the highest rate
* Load limits on currency pair change
* Show limits for lowest min provider
* Show limits for lowest min and highest max
* Sync best rate every 10 seconds instead of calculating it on every amount change
* Fix conflicts with flutter upgrade
Add null safety to changes in exchange_view_model.dart
* Remove un-necessary checks
