feat(rum): report app launch (TTID) on Android

[Fiona2016]

What

Flutter never reported an app launch on Android. RumAppStartupDetector registers its Activity lifecycle callbacks when the RUM feature initializes, and Flutter initializes the SDK from Dart main() — by then the first Activity has already been created, so the detector never sees the launch.

The launch is now reported from the plugin, on the launch frame.

How

• The plugin asks on every Android launch and the native SDK decides. Whether the native detector saw the launch is not something a host can tell: initializing the native SDK before Flutter attaches does not mean it initialized early enough to observe the first Activity, so an early version of this PR — which only asked when Flutter owned the initialization — lost the launch entirely on hosts that initialize late. notifyAppLaunchIfAbsent (feat(rum): let cross-platform hosts report an app launch the native detector misses fc-sdk-android#23) arbitrates instead: the detector wins whenever it fired, since it always fires from the first Activity's pre-create, before any frame a host could render. It also de-duplicates several engines in one process, so the plugin keeps no latch of its own.
• DatadogSdkPlugin is now ActivityAware and stamps the UI creation time from onAttachedToActivity. Engine attach is deliberately not used: a pre-warmed or headless engine attaches with no UI, which would place the launch start far too early. An engine that never gets an Activity reports no UI creation time, and the native SDK falls back to timing from process start. The stamp uses AtomicLong.compareAndSet so two engines attaching concurrently cannot race.
• Dart passes how long ago the launch frame finished rasterizing, so neither the callback scheduling nor the method channel round trip is counted as launch time. Measured at ~61ms on a release build. Timeline.now shares its clock with FrameTiming, which is what makes the subtraction meaningful; an implausible reading falls back to 0.
• Nothing is reported when the first frame is already on screen when RUM is enabled. addTimingsCallback only delivers later frames, so the next one is not the launch frame — and a static page may never render another at all. Reporting nothing beats reporting something wrong.
• Registration tolerates a missing binding, since enable() is reachable from plain Dart contexts (unit tests, background isolates) where touching SchedulerBinding.instance throws rather than returning null.

iOS measures app launch natively and is unchanged (no-op).

Also fixed here

• attachToExisting reports a refresh rate again. It passed a null vital frequency, so the callback was never registered even with reportFlutterPerformance on — a regression against the behaviour before build and refresh-rate timings were split apart. DatadogAttachConfiguration now carries vitalUpdateFrequency, defaulting to VitalsFrequency.average.
• One timings callback instead of two. Performance and refresh-rate reporting stay independently configurable but now share a single SchedulerBinding subscription and send one platform message per frame batch.

Depends on

• cloud.flashcat:dd-sdk-android-rum 0.5.0 for notifyAppLaunchIfAbsent — feat(rum): let cross-platform hosts report an app launch the native detector misses fc-sdk-android#23. ext.datadog_version is bumped here.
• Stacked on feat(rum): report Flutter view refresh rate on Android via external hook #11 (refresh rate); base is that branch, retarget to publish once it merges.

Testing

• Dart suite: 260 tests pass.
• Plugin Kotlin suite: 74 tests pass (run against a locally published 0.5.0; this caught a leftover reference to the removed plugin latch that no Dart test or flutter analyze could reach).
• flutter analyze lib clean.
• Verified end to end against a local RUM intake and the dev Doris cluster: startup_type=cold_start, app_launch_metric=ttid, durations 0.6s–2.1s, landing in t_vitals. Cross-checked against the system's own Fully drawn figure for the same launches.

Known and intentional: a hot start (process alive, Activity alive, returning from background) reports nothing — tracked separately.

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