Feature/jrc 8103.fix.api.34.heuristic

[jrodiz]

firebase-perf: fix API 34+ background-start heuristic in AppStartTrace

Fixes #8103 (follow-up to b/339891952).

AppStartTrace's background-start detection relied on the ordering of a posted main-thread runnable vs the first onActivityCreated. On API 34+ the OS can drain the runnable before delivering the activity-launch Binder transaction even on a genuine cold foreground launch — the gap on real Pixel devices is ~200–300 ms, well above the previous 50 ms tolerance. Every affected user lost their _app_start traces.

Fix

On API 34+, resolveIsStartedFromBackground switches from the timing-window heuristic to an OS-reported process start cause. Pre-API-34 is unchanged.

API <34: legacy mainThreadRunnableTime check (unchanged).
API 34+: AppStartCause.cause
         FOREGROUND ⇒ log.
         UNKNOWN / null ⇒ suppress.

The new AppStartCause helper reads RunningAppProcessInfo.importance via ActivityManager.getMyMemoryState once, at first capture inside AppStartTrace.registerActivityLifecycleCallbacks. IMPORTANCE_FOREGROUND ⇒ FOREGROUND (activity-driven start); anything else ⇒ UNKNOWN. Pre-API-34 returns UNKNOWN so the legacy decision stays in charge. FirebasePerfEarly stops scheduling StartFromBackgroundRunnable on API 34+ since its output is no longer consumed.

Behavior matrix

Scenario	API <34	API 34+
Foreground launcher tap	log (unchanged)	log via FOREGROUND cause
Background broadcast (no activity)	n/a	n/a
Warm start (broadcast → activity)	suppress (unchanged)	suppress via UNKNOWN cause

Tests

./gradlew :firebase-perf:spotlessApply :firebase-perf:testReleaseUnitTest — 0 failures. AppStartTraceTest 9/9 (@Config(sdk = 33) for the pre-API-34 path; FOREGROUND / UNKNOWN / null for API 34+). AppStartCauseTest 6/6 (null-context, pre-API-34, API 34 FOREGROUND / SERVICE / CACHED importance branches, constructor).
Test project app: https://github.com/jrodiz/firebase-perf-e2e-for-issue8103

Risks

• API 34+ behavior changes for every app: timing-window heuristic gone, importance-based signal wired in. _app_start emission rate rises on the previously-broken population — the intent.
• RunningAppProcessInfo.importance is stable public SDK and a single Binder IPC; no reflection.
• Pre-API-34 path untouched.

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