Fix nullness warnings.

These two things were reported by the nullness checker as warnings.

PiperOrigin-RevId: 348650535

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/CachedContentIndex.java

@@ -38,6 +38,7 @@ import com.google.android.exoplayer2.util.Assertions;
 import com.google.android.exoplayer2.util.AtomicFile;
 import com.google.android.exoplayer2.util.ReusableBufferedOutputStream;
 import com.google.android.exoplayer2.util.Util;
+import com.google.common.collect.ImmutableSet;
 import java.io.BufferedInputStream;
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
@@ -301,9 +302,8 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
 
   /** Removes all resources whose {@link CachedContent CachedContents} are empty and unlocked. */
   public void removeEmpty() {
-    String[] keys = new String[keyToContent.size()];
-    keyToContent.keySet().toArray(keys);
-    for (String key : keys) {
+    // Create a copy of the keys as the underlying map is modified by maybeRemove(key).
+    for (String key : ImmutableSet.copyOf(keyToContent.keySet())) {
       maybeRemove(key);
     }
   }

library/core/src/main/java/com/google/android/exoplayer2/video/VideoFrameReleaseHelper.java

@@ -409,7 +409,7 @@ public final class VideoFrameReleaseHelper {
 
     private final Handler handler;
     private final HandlerThread choreographerOwnerThread;
-    @MonotonicNonNull private Choreographer choreographer;
+    private @MonotonicNonNull Choreographer choreographer;
     private int observerCount;
 
     public static VSyncSampler getInstance() {