Address PR review

Author: romtsn

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

@@ -183,13 +183,15 @@ private void registerReceiver(
   }
 
   private void unregisterReceiver() {
+    final @Nullable SystemEventsBroadcastReceiver receiverRef;
     try (final @NotNull ISentryLifecycleToken ignored = receiverLock.acquire()) {
       isStopped = true;
+      receiverRef = receiver;
+      receiver = null;
     }
 
-    if (receiver != null) {
-      context.unregisterReceiver(receiver);
-      receiver = null;
+    if (receiverRef != null) {
+      context.unregisterReceiver(receiverRef);
     }
   }
 

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

@@ -25,7 +25,6 @@ import io.sentry.protocol.SentryId
 import io.sentry.protocol.SentryTransaction
 import io.sentry.transport.RateLimiter
 import org.junit.runner.RunWith
-import org.mockito.kotlin.mock
 import org.robolectric.annotation.Config
 import java.util.LinkedList
 import kotlin.test.BeforeTest