fix(android): Preserve app start activity counter

Keep the foreground headless guard from faking an observed activity so
late standalone app start init still lets the first real activity classify
startup and reset warm-start state correctly.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: buenaflor

sentry-android-core/src/main/java/io/sentry/android/core/performance/AppStartMetrics.java

@@ -479,12 +479,9 @@ private void handleHeadlessAppStartIfNeededOnMain() {
     if (activeActivitiesCounter.get() == 0) {
       // SDK init happened after Application.onCreate (e.g. deferred/late init inside an Activity):
       // we missed the Activity's onActivityCreated, but a foreground process means it was a real
-      // launch, not a headless start. Don't emit a (fake) headless app-start; just keep the counter
-      // consistent so the later onActivityDestroyed doesn't go negative. Gated on the listener so
-      // only the standalone-app-start path (which is what could emit a headless transaction) is
-      // affected.
+      // launch, not a headless start. Gated on the listener so only the standalone-app-start path
+      // (which is what could emit a headless transaction) is affected.
       if (headlessAppStartListener != null && ContextUtils.isForegroundImportance()) {
-        activeActivitiesCounter.compareAndSet(0, 1);
         return;
       }
 
@@ -629,7 +626,11 @@ public void onActivityStopped(@NonNull Activity activity) {
   public void onActivityDestroyed(@NonNull Activity activity) {
     CurrentActivityHolder.getInstance().clearActivity(activity);
 
-    final int remainingActivities = activeActivitiesCounter.decrementAndGet();
+    int remainingActivities = activeActivitiesCounter.decrementAndGet();
+    if (remainingActivities < 0) {
+      activeActivitiesCounter.set(0);
+      remainingActivities = 0;
+    }
     // if the app is moving into background
     // as the next onActivityCreated will treat it as a new warm app start
     if (remainingActivities == 0 && !activity.isChangingConfigurations()) {

sentry-android-core/src/test/java/io/sentry/android/core/performance/AppStartMetricsTest.kt

@@ -264,6 +264,19 @@ class AppStartMetricsTest {
 
     assertEquals(0, listenerCalls.get())
     assertEquals(AppStartMetrics.AppStartType.UNKNOWN, metrics.appStartType)
+
+    SystemClock.setCurrentTimeMillis(SystemClock.uptimeMillis() + 100)
+    val activity = mock<Activity>()
+    whenever(activity.isChangingConfigurations).thenReturn(false)
+    metrics.onActivityCreated(activity, null)
+
+    assertEquals(AppStartMetrics.AppStartType.WARM, metrics.appStartType)
+
+    metrics.onActivityDestroyed(activity)
+    SystemClock.setCurrentTimeMillis(SystemClock.uptimeMillis() + 100)
+    metrics.onActivityCreated(mock<Activity>(), null)
+
+    assertEquals(AppStartMetrics.AppStartType.WARM, metrics.appStartType)
   }
 
   @Test