fix: Don't use computeIfAbsent in PerContextEventSummarizer

[abelonogov-ld]

Requirements

• I have added test coverage for new or changed functionality (behavior is unchanged; covered by the existing tests)
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

The Android client SDK supports API 21 and up, and summarizes per context with PerContextEventSummarizer.

Describe the solution you've provided

Android below API 24 has neither Map.computeIfAbsent nor java.util.function. D8 turns the EventSummarizer::new reference into a synthetic Function, so the first flag evaluation in a client-side SDK on API 21–23 throws NoClassDefFoundError.

summarizeEvent now does a get and, on a miss, a put, which behaves the same. The existing PerContextEventSummarizerTest (18 tests) and EventSummarizerTest (7 tests) pass unchanged.

Describe alternatives you've considered

Having the Android SDK fall back to the aggregated EventSummarizer below API 24. That works around it for one consumer, but leaves the shared class unusable on those API levels.

Additional context

No public API change.

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Note

Overview
Fixes client-side event summarization on Android API 21–23, where the first flag evaluation could crash with NoClassDefFoundError because Map.computeIfAbsent and java.util.function are unavailable and D8 turns EventSummarizer::new into a synthetic Function.

PerContextEventSummarizer.summarizeEvent now uses get / create / put instead of computeIfAbsent, with the same per-context summarizer behavior. No public API change; existing tests cover the behavior.

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