RUM-4329: Fix ConcurrentModificationException during features iteration

[0xnm]

What does this PR do?

It is possible to have a crash like that:

java.util.ConcurrentModificationException
	at java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:760)
	at java.util.LinkedHashMap$LinkedEntryIterator.next(LinkedHashMap.java:792)
	at java.util.LinkedHashMap$LinkedEntryIterator.next(LinkedHashMap.java:790)
	at com.datadog.android.core.DatadogCore.updateFeatureContext(SourceFile:65)
	at com.datadog.android.rum.internal.domain.scope.RumViewScope.<init>(SourceFile:21)
	at com.datadog.android.rum.internal.domain.scope.RumViewScope.<init>(SourceFile:31)
	at com.datadog.android.rum.internal.domain.scope.RumViewManagerScope.handleEvent(SourceFile:620)
	at com.datadog.android.rum.internal.domain.scope.RumSessionScope.handleEvent(SourceFile:246)
	at com.datadog.android.rum.internal.domain.scope.RumApplicationScope.handleEvent(SourceFile:356)
	at androidx.lifecycle.DispatchQueue$$ExternalSyntheticLambda0.run(SourceFile:278)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:487)
	at java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:644)
	at java.lang.Thread.run(Thread.java:1012)

The problem is that DatadogCore.features can be accessed from different threads. This PR makes access for the items of this collection thread-safe.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 83.69%. Comparing base (0c3e683) to head (29df747).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2012      +/-   ##
===========================================
- Coverage    83.76%   83.69%   -0.07%     
===========================================
  Files          488      488              
  Lines        17779    17778       -1     
  Branches      2667     2667              
===========================================
- Hits         14892    14879      -13     
- Misses        2174     2178       +4     
- Partials       713      721       +8     

Files	Coverage Δ
...ain/kotlin/com/datadog/android/core/DatadogCore.kt	81.98% <100.00%> (ø)

... and 23 files with indirect coverage changes

[xgouchet]

Note

NitPick: it'd be nice to write a test that reproduces this issue, and then ensure the fix fixes it

[0xnm]

@xgouchet fair point, I've updated my PR with test.