Fixes #5439: Fix ConcurrentModificationException

[tanyagoyan-boop]

Problem

The listeners in ConfigRealtimeHandler are stored in a LinkedHashSet. If a listener calls remove() inside its own onError, ConfigRealtimeHttpClient.propagateErrors throws a ConcurrentModificationException while looping over the set. On the main thread (Activity onResume → setBackgroundState(false) → makeRealtimeHttpConnection), the app crashes with "Unable to resume activity".

Fix

Store the listeners in a CopyOnWriteArraySet. Loops then run over a snapshot of the set, so a listener can remove itself mid-loop without breaking it.

Testing

• Added ConfigRealtimeHandlerTest: listener A removes itself in onError, and the test checks that listener B is still notified. It fails before the fix and passes after.
  With LinkedHashSet : https://screenshot-v2.corp.google.com/43mnvd33td1bg
  With CopyOnWriteArraySet : https://screenshot-v2.corp.google.com/4sl8fqrodc3h8
• Manual repro in the Remote Config quickstart, using a local SDK build with retries lowered to 1 and the Realtime API disabled:
  • Before the fix, :

Beforefix.mov

• After the fix, both listeners received onError and there was no crash.

AfterFix.mov

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[github-actions]

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[rlazo]

Duplicated section

[tanyagoyan-boop]

fixed.

[rlazo]

Suggested change

	- [fixed] `ConcurrentModificationException` crash when a config update listener is removed
	- [fixed] Fixed a `ConcurrentModificationException` crash when a config update listener is removed
	from inside its own callback. (#5439)

[tanyagoyan-boop]

fixed

[rlazo]

Suggested change

	- [fixed] Fixed a `ConcurrentModificationException` crash when a config update listener is removed from inside its own callback. (#5439)
	- [fixed] Fixed a `ConcurrentModificationException` crash when a config update listener is removed from inside its own callback. (#8686)

[tanyagoyan-boop]

fixed