[Update task] use onBackPressedDispatcher callbacks instead of overriding Activity's onBackPressed() (deprecated method)

[mzorz]

See:

• NoteEditorActivity
• NewCredentialsActivity

Use new onBackPressedDispatcher callbacks instead.

[dangermattic]

Thanks for reporting! 👍

[mzorz]

Closing via 22fefde in #1607