[flutter_form_bloc] upgrades intl to 0.19.0 + passes "dart analyze"

[enzo-santos]

This PR only affects flutter_form_bloc. It

• upgrades intl package to 0.19.0, matching the Flutter pinned version for it
• replaces deprecated methods/fields with their active counterparts
  • MaterialStateProperty -> WidgetStateProperty
  • ColorScheme.background -> ColorScheme.surface
  • WillPopScope -> PopScope
• replaces private State concrete types with their respective public API ("Avoid using private types in public APIs")
• checks for context.mounted after asynchronous calls ("Do not use BuildContext across asynchronous gaps")

[oengmengthong]

hope owner will approve merge

[vasilich6107]

This package does not seem to be supported
Just a friendly reminder to migrate

#312 (comment)