fix(extend-app-start): Make shouldSendStartMeasurements volatile

It is written without a lock in onAppStartSpansSent() and read across threads in canExtendAppStart() (via extendAppStart()), with no happens-before edge. volatile guarantees visibility, matching the AtomicInteger/AtomicBoolean siblings in the same check.

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

Author: buenaflor

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

@@ -87,7 +87,7 @@ public enum AppStartType {
   private @Nullable IContinuousProfiler appStartContinuousProfiler = null;
   private @Nullable TracesSamplingDecision appStartSamplingDecision = null;
   private boolean isCallbackRegistered = false;
-  private boolean shouldSendStartMeasurements = true;
+  private volatile boolean shouldSendStartMeasurements = true;
   private final AtomicInteger activeActivitiesCounter = new AtomicInteger();
   private final AtomicBoolean firstDrawDone = new AtomicBoolean(false);
   private final AtomicBoolean headlessAppStartCheckPending = new AtomicBoolean(false);