firebase-perf: document why the importance signal is gated to API 34+

[jrodiz]

Answers #8509 (follow-up question to #8103 / PR #8326). Documentation only: no behavior change, no API change, no CHANGELOG entry.

Why

AppStartCause.capture() reads RunningAppProcessInfo.importance on every API level but classifies on it only from API 34 up. The comment at the gate said what happened (// API < 34: legacy AppStartTrace logic owns the decision.) and not why, leaving the reasoning only in the PR body for #8326. This moves it to the gate.

Changes

AppStartCause.java: class javadoc notes that the importance read isn't version-gated (getMyMemoryState is API 16+) and only the classification is; capture()'s javadoc records the rationale; the inline comment notes that importance is recorded below 34 so the two signals can be compared before any future tier flip.

AppStartTrace.java: resolveIsStartedFromBackground() points at AppStartCause#capture and references #8509 alongside #8103.

The rationale recorded

1. No pre-API-34 defect. [firebase-perf] _app_start suppressed on API >=34 for large apps — 50ms MAX_BACKGROUND_RUNNABLE_DELAY threshold insufficient (b/339891952 follow-up) #8103 is an API-34+ ordering change; below 34 the legacy check classifies correctly (API 33 launcher tap: activity.resumed 84 ms, runnable 101 ms).
2. The signals aren't equivalent. PROCESS_STATE_BOUND_TOP maps to IMPORTANCE_FOREGROUND, so a process forked by a foreground app binding one of its services samples FOREGROUND, and a warm start that follows would be admitted as _app_start, which the pre-34 path suppresses today.
3. Importance-driven suppression fails silently, where the legacy check fails the other way. On 34+ that trade beat losing the trace entirely; below 34 there's nothing on the other side of it.
4. The procState → importance mapping isn't one function pre-34. procStateToImportanceForTargetSdk returns the *_PRE_26 / *_PRE_28 constants below targetSdk 26, and minSdk here is 23.
5. No production data backs the swap on that population: the 34+ flip rests on four emulator images across three scenarios, plus the device reports in [firebase-perf] _app_start suppressed on API >=34 for large apps — 50ms MAX_BACKGROUND_RUNNABLE_DELAY threshold insufficient (b/339891952 follow-up) #8103 and Firebase Performance does not send app start traces on some devices (API 34 Pixel 7) #5920.

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