perf(android): Defer Session Replay start off SDK init critical path

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Performance
+
+- Defer starting Session Replay off the `Sentry.init` critical path to reduce app start time ([#5904](https://github.com/getsentry/sentry-java/pull/5904))
+
 ### Fixes
 
 - Clear contexts when calling `Scope.clear()` ([#5902](https://github.com/getsentry/sentry-java/pull/5902))

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

@@ -3,6 +3,8 @@
 import android.annotation.SuppressLint;
 import android.app.Application;
 import android.content.Context;
+import android.os.Handler;
+import android.os.Looper;
 import android.os.Process;
 import android.os.SystemClock;
 import android.os.Trace;
@@ -203,7 +205,11 @@ public static void init(
             scopes.startSession();
           }
         }
-        scopes.getOptions().getReplayController().start();
+        // Defer starting replay off the SDK init critical path so it doesn't add to app start
+        // time. start() is idempotent, so the later start() from the app lifecycle integration
+        // (once the first activity is in foreground) is a no-op if this one ran first.
+        new Handler(Looper.getMainLooper())
+            .post(() -> scopes.getOptions().getReplayController().start());
       }
     } catch (IllegalAccessException e) {
       logger.log(SentryLevel.FATAL, "Fatal error during SentryAndroid.init(...)", e);

sentry-android-core/src/test/java/io/sentry/android/core/SentryAndroidTest.kt

@@ -352,6 +352,8 @@ class SentryAndroidTest {
   @Config(sdk = [26])
   fun `init starts session replay if app is in foreground`() {
     initSentryWithForegroundImportance(true) { _ ->
+      // replay start is posted to the main looper, so drain it before asserting
+      Shadows.shadowOf(Looper.getMainLooper()).idle()
       assertTrue(Sentry.getCurrentHub().options.replayController.isRecording())
     }
   }