Add missing synchronized keywords and assertions

applyContentMetadataMutations and getContentMetadata methods suppossed to be synchronized and assert the instance isn't released.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191419637

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

@@ -306,14 +306,16 @@ public final class SimpleCache implements Cache {
   }
 
   @Override
-  public void applyContentMetadataMutations(String key, ContentMetadataMutations mutations)
-      throws CacheException {
+  public synchronized void applyContentMetadataMutations(
+      String key, ContentMetadataMutations mutations) throws CacheException {
+    Assertions.checkState(!released);
     index.applyContentMetadataMutations(key, mutations);
     index.store();
   }
 
   @Override
-  public ContentMetadata getContentMetadata(String key) {
+  public synchronized ContentMetadata getContentMetadata(String key) {
+    Assertions.checkState(!released);
     return index.getContentMetadata(key);
   }