fix: replace deprecated elements with equivalent up-to-date items in at_commons_flutter

[srieteja]

Closes #842

- What I did

• Replaced deprecated elements inat_commons_flutter's example app with up-to-date design elements

- How I did it

• BackgroundColor in MaterialApp() was previously deprecated and now removed. The latest version's equivalent of BackgroundColor is called 'surface' which is now used in the examples.

- How to verify it

• Static analysis should pass. Also, requires manual testing to ensure design elements are not broken

- Description for the changelog

fix: replace deprecated elements with equivalent up-to-date items in at_commons_flutter

[cpswan]

@srieteja have you done the manual testing, or does that need somebody else to step in?

[srieteja]

@cpswan I've just setup my emulators. Will be doing the testing now

[cpswan]

@CurtlyCritchlow 👀 can you please sanity check this (manual tests to ensure layout isn't broken)

[srieteja]

I've completed the manual testing for this one. I'm not sure if this can be merged yet. We can proceed with one of the following actions:

1. We merge this since all the stable static analyses are passing (or)
2. We wait for this merge until Clear up info messages from flutter analyze #847 is closed (which would ensure all the beta channel static analyses pass too)

@cpswan need your input on this.