Clear up info messages from flutter analyze

[cpswan]

Which at_widget package you find issue with?

at_backupkey_flutter, at_chat_flutter, at_common_flutter, at_contacts_flutter, at_contacts_group_flutter, at_events_flutter, at_follows_flutter, at_invitation_flutter, at_notify_flutter, at_onboarding_flutter, at_sync_ui_flutter, at_theme_flutter

On what platforms did you experience the issue?

Linux

What version are you using?

Latest

What happened?

In #846 we made the call to enable infos on beta channel so we get early warning of deprecation. This ticket is to clear down existing tech debt.

Steps to reproduce

1. Go to any of the listed packages
2. Run `flutter analyze`

Additional info

#842 and #843 take priority, as they're already showing issues in stable channel. This should help us avoid similar problems in the future.

Flutter analyze output

An example:

Analyzing at_backupkey_flutter...                               

   info • 'background' is deprecated and shouldn't be used. Use surface instead. This feature was deprecated after v3.18.0-0.1.pre • example/lib/main.dart:57:19 • deprecated_member_use
   info • 'background' is deprecated and shouldn't be used. Use surface instead. This feature was deprecated after v3.18.0-0.1.pre • example/lib/main.dart:66:19 • deprecated_member_use

[srieteja]

Implemented changes to clear out the issues reported by flutter analyze. Carrying it forward to PR88 for manually testing each of these example apps/apps.