Migrate BackPress to OnBackPressedCallback

[anandwana001]

Fixes #2944

super.onBackPressed() is now deprecated, and documentation suggested using OnBackPressedCallback

Back Press should work correctly.

@shobhitagarwal1612 PTAL?

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 62.54%. Comparing base (b660181) to head (72e026e).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...ain/java/com/google/android/ground/MainActivity.kt	42.85%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2945      +/-   ##
============================================
- Coverage     62.54%   62.54%   -0.01%     
  Complexity     1203     1203              
============================================
  Files           267      267              
  Lines          6357     6362       +5     
  Branches        879      880       +1     
============================================
+ Hits           3976     3979       +3     
- Misses         1831     1833       +2     
  Partials        550      550              

Files with missing lines	Coverage Δ
...ain/java/com/google/android/ground/MainActivity.kt	48.38% <42.85%> (+0.65%)	⬆️

[shobhitagarwal1612]

Can you please update description & specify how the change was verified?