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

[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, and 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

Replace the timing-window heuristic in resolveIsStartedFromBackground on API 34+ with an OS-reported process start cause. Pre-API-34 keeps the legacy runnable-before-onActivityCreated check.

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

New ProcessStartCause helper reads RunningAppProcessInfo.importance at first capture via ActivityManager.getMyMemoryState. IMPORTANCE_FOREGROUND indicates an activity-driven start; anything else maps to UNKNOWN. Pre-API-34 returns UNKNOWN and the legacy logic owns the decision.

Captured once in AppStartTrace.registerActivityLifecycleCallbacks and consumed by the decision only. No new custom attributes are emitted on the _experiment_app_start_ttid trace.

FirebasePerfEarly stops scheduling StartFromBackgroundRunnable on API 34+ since its output is no longer consumed there.

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 — clean.
• ./gradlew :firebase-perf:testReleaseUnitTest — 0 failures / 0 errors.
  • AppStartTraceTest: 9/9 — @Config(sdk = 33) regression tests for the pre-API-34 path; FOREGROUND / UNKNOWN / null-cause cases for API 34+.
  • ProcessStartCauseTest: 6/6 — null-context, pre-API-34, API 34 importance branches (FOREGROUND / SERVICE / CACHED), constructor.

Files

File	Change
firebase-perf/src/main/java/.../FirebasePerfEarly.java	Skip StartFromBackgroundRunnable schedule on API 34+.
firebase-perf/src/main/java/.../metrics/AppStartTrace.java	New processStartCause field; new resolveIsStartedFromBackground decision split.
firebase-perf/src/main/java/.../metrics/ProcessStartCause.java	New file. Importance-only capture, ~100 lines.
firebase-perf/src/test/java/.../metrics/AppStartTraceTest.java	New per-API regression and causal-signal tests.
firebase-perf/src/test/java/.../metrics/ProcessStartCauseTest.java	New file.
firebase-perf/CHANGELOG.md	[fixed] entry referencing #8103.

Risks

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

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