Add flag to CachedContentIndex to disable encryption. This allows the encryption feature

to be disabled gracefully: encrypted index files may be read, but plaintext will be written.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=165196508

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

@@ -64,6 +64,7 @@ import javax.crypto.spec.SecretKeySpec;
   private final AtomicFile atomicFile;
   private final Cipher cipher;
   private final SecretKeySpec secretKeySpec;
+  private final boolean encrypt;
   private boolean changed;
   private ReusableBufferedOutputStream bufferedOutputStream;
 
@@ -80,10 +81,21 @@ import javax.crypto.spec.SecretKeySpec;
    * Creates a CachedContentIndex which works on the index file in the given cacheDir.
    *
    * @param cacheDir Directory where the index file is kept.
-   * @param secretKey If not null, cache keys will be stored encrypted on filesystem using AES/CBC.
+   * @param secretKey 16 byte AES key for reading and writing the cache index.
-   *     The key must be 16 bytes long.
    */
   public CachedContentIndex(File cacheDir, byte[] secretKey) {
+    this(cacheDir, secretKey, secretKey != null);
+  }
+
+  /**
+   * Creates a CachedContentIndex which works on the index file in the given cacheDir.
+   *
+   * @param cacheDir Directory where the index file is kept.
+   * @param secretKey 16 byte AES key for reading, and optionally writing, the cache index.
+   * @param encrypt When false, a plaintext index will be written.
+   */
+  public CachedContentIndex(File cacheDir, byte[] secretKey, boolean encrypt) {
+    this.encrypt = encrypt;
     if (secretKey != null) {
       Assertions.checkArgument(secretKey.length == 16);
       try {
@@ -288,10 +300,11 @@ import javax.crypto.spec.SecretKeySpec;
       output = new DataOutputStream(bufferedOutputStream);
       output.writeInt(VERSION);
 
-      int flags = cipher != null ? FLAG_ENCRYPTED_INDEX : 0;
+      boolean writeEncrypted = encrypt && cipher != null;
+      int flags = writeEncrypted ? FLAG_ENCRYPTED_INDEX : 0;
       output.writeInt(flags);
 
-      if (cipher != null) {
+      if (writeEncrypted) {
         byte[] initializationVector = new byte[16];
         new Random().nextBytes(initializationVector);
         output.write(initializationVector);

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

@@ -60,10 +60,24 @@ public final class SimpleCache implements Cache {
    *     The key must be 16 bytes long.
    */
   public SimpleCache(File cacheDir, CacheEvictor evictor, byte[] secretKey) {
+    this(cacheDir, evictor, secretKey, secretKey != null);
+  }
+
+  /**
+   * Constructs the cache. The cache will delete any unrecognized files from the directory. Hence
+   * the directory cannot be used to store other files.
+   *
+   * @param cacheDir A dedicated cache directory.
+   * @param evictor The evictor to be used.
+   * @param secretKey If not null, cache keys will be stored encrypted on filesystem using AES/CBC.
+   *     The key must be 16 bytes long.
+   * @param encrypt When false, a plaintext index will be written.
+   */
+  public SimpleCache(File cacheDir, CacheEvictor evictor, byte[] secretKey, boolean encrypt) {
     this.cacheDir = cacheDir;
     this.evictor = evictor;
     this.lockedSpans = new HashMap<>();
-    this.index = new CachedContentIndex(cacheDir, secretKey);
+    this.index = new CachedContentIndex(cacheDir, secretKey, encrypt);
     this.listeners = new HashMap<>();
     // Start cache initialization.
     final ConditionVariable conditionVariable = new ConditionVariable();