Replace CachedContentIndex usage of Random with SecureRandom

This is used to generate the initialization vector for encrypting the
cache contents.

Startblock:
   <unknown commit> is submitted
   and then
   3w have passed
PiperOrigin-RevId: 303932151

RELEASENOTES.md

@@ -61,6 +61,8 @@
         `DecoderVideoRenderer` and `DecoderAudioRenderer` respectively, and
         generalized to work with `Decoder` rather than `SimpleDecoder`.
     *   Add media item based playlist API to Player.
+    *   Update `CachedContentIndex` to use `SecureRandom` for generating the
+        initialization vector used to encrypt the cache contents.
 *   Text:
     *   Parse `<ruby>` and `<rt>` tags in WebVTT subtitles (rendering is coming
         later).

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

@@ -45,11 +45,11 @@ import java.io.OutputStream;
 import java.security.InvalidAlgorithmParameterException;
 import java.security.InvalidKeyException;
 import java.security.NoSuchAlgorithmException;
+import java.security.SecureRandom;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.Map;
-import java.util.Random;
 import java.util.Set;
 import javax.crypto.Cipher;
 import javax.crypto.CipherInputStream;
@@ -494,7 +494,7 @@ import org.checkerframework.checker.nullness.compatqual.NullableType;
     private final boolean encrypt;
     @Nullable private final Cipher cipher;
     @Nullable private final SecretKeySpec secretKeySpec;
-    @Nullable private final Random random;
+    @Nullable private final SecureRandom random;
     private final AtomicFile atomicFile;
 
     private boolean changed;
@@ -517,7 +517,7 @@ import org.checkerframework.checker.nullness.compatqual.NullableType;
       this.encrypt = encrypt;
       this.cipher = cipher;
       this.secretKeySpec = secretKeySpec;
-      random = encrypt ? new Random() : null;
+      random = encrypt ? new SecureRandom() : null;
       atomicFile = new AtomicFile(file);
     }