Some additional accessibility test coverage

[jonathanmos]

What does this PR do?

Refactor DefaultAccessibilitySnapshotManager to return a nullable object - this simplifies the later logic in terms of knowing if anything changed in the accessibility state.

Adds some additional test coverage for accessibility around the logic for view reduction

Motivation

What inspired you to submit this pull request?

Additional Notes

Anything else we should know when reviewing?

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.01%. Comparing base (207c199) to head (aa79b82).
⚠️ Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
.../android/rum/internal/domain/scope/RumViewScope.kt	88.89%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2823      +/-   ##
===========================================
- Coverage    70.03%   70.01%   -0.01%     
===========================================
  Files          845      845              
  Lines        31497    31467      -30     
  Branches      5283     5286       +3     
===========================================
- Hits         22056    22031      -25     
+ Misses        7962     7957       -5     
  Partials      1479     1479              

Files with missing lines	Coverage Δ
...kotlin/com/datadog/android/rum/RumConfiguration.kt	97.44% <ø> (ø)
...tadog/android/rum/internal/domain/RumDataWriter.kt	100.00% <100.00%> (+1.82%)	⬆️
...cessibility/DefaultAccessibilitySnapshotManager.kt	100.00% <100.00%> (ø)
.../android/rum/internal/domain/scope/RumViewScope.kt	94.28% <88.89%> (-0.43%)	⬇️

... and 36 files with indirect coverage changes

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