feat(android): Register StandaloneAppStart SDK integration marker

Advertise that standalone app start tracing is active by adding a
StandaloneAppStart marker to the SDK metadata integrations when the
feature is enabled.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: buenaflor

CHANGELOG.md

@@ -10,6 +10,7 @@
   - The standalone transaction shares the same `traceId` as the first `ui.load` activity transaction so they remain linked in the trace view
   - Also covers non-activity starts (broadcast receivers, services, content providers)
   - On Android 15+ (API 35), the standalone transaction reports why the OS started the process via `app.vitals.start.reason` trace data (e.g. `launcher`, `broadcast`, `service`, `content_provider`), derived from `ApplicationStartInfo.getReason()` ([#5552](https://github.com/getsentry/sentry-java/pull/5552))
+  - When enabled, registers a `StandaloneAppStart` marker in the SDK metadata `integrations` ([#5552](https://github.com/getsentry/sentry-java/pull/5552))
 
 ### Improvements
 

sentry-android-core/src/main/java/io/sentry/android/core/ActivityLifecycleIntegration.java

@@ -141,6 +141,7 @@ public void register(final @NotNull IScopes scopes, final @NotNull SentryOptions
 
     if (performanceEnabled && this.options.isEnableStandaloneAppStartTracing()) {
       AppStartMetrics.getInstance().setHeadlessAppStartListener(this::onHeadlessAppStart);
+      addIntegrationToSdkVersion("StandaloneAppStart");
     }
 
     this.options.getLogger().log(SentryLevel.DEBUG, "ActivityLifecycleIntegration installed.");