[Feature] Prevent screen reader from focusing content behind Snackbar

[TimRoe]

Description of Change

Added two props to the outermost container of the Snackbar display, one for each iOS and Android. Both effectively say the element should be focusable over other views (e.g. the ones below them).

Also added story documentation/a11y tweaks I'd made for a prior ticket but forgot to commit before merging.

Testing Packages

• 0.21.1-alpha.5

Screenshots/Video

iOS (first with fix commented out demoing the tap through the Snackbar then "Refresh" around 16 seconds is with fix enabled):

RPReplay_Final1723051194.MP4

Android (first with fix commented out demoing the tap through the Snackbar then around 17 seconds is with fix enabled):

Screen_Recording_20240807-132533_VA.mp4

Testing

• Tested on iOS
• Tested on Android
• Not tested on Web
  • We don't test screen readers on web as an unsupported platform
  • The way the Snackbar story is configured for web wouldn't encounter the issue
  • The props used are iOS/Android specific and should not do anything on web

PR Checklist

Code reviewer validation:

• General
  • PR is linked to ticket(s)
  • PR has changelog label applied if it's to be included in the changelog
  • Acceptance criteria:
    • All satisfied or
    • Documented reason for not being performed or
    • Split to separate ticket and ticket is linked by relevant AC(s)
  • Above PR sections adequately filled out
  • If any breaking changes, in accordance with the pre-1.0.0 versioning guidelines: a CU ticket has been created for the VA Mobile App detailing necessary adjustments with the package version that will be published by this ticket
• Code
  • Tests are included if appropriate (or split to separate ticket)
  • New functions have proper TSDoc annotations

Publish

While changes warrant a new version per the versioning guidelines, we are postponing publishing Snackbar until complete.