fix(feedback): clear stale activity ref and reset shake state on stop

- Clear currentActivity in onActivityDestroyed to prevent holding a
  stale reference to a destroyed activity context
- Reset shakeCount and firstShakeTimestamp in stop() to prevent
  cross-session false triggers across pause/resume cycles

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: antonis

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

@@ -105,6 +105,7 @@ public void onActivityDestroyed(final @NotNull Activity activity) {
     // outlive its host activity being destroyed.
     if (isDialogShowing && activity == currentActivity) {
       isDialogShowing = false;
+      currentActivity = null;
       if (options != null) {
         options.getFeedbackOptions().setOnFormClose(previousOnFormClose);
       }

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

@@ -91,6 +91,8 @@ public void start(final @NotNull Context context, final @NotNull Listener shakeL
 
   public void stop() {
     listener = null;
+    shakeCount = 0;
+    firstShakeTimestamp = 0;
     if (sensorManager != null) {
       sensorManager.unregisterListener(this);
     }