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. Internal SDK metadata only.

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

Author: buenaflor

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.");