Implement CacheContentIndex storage switching

This change enables transitioning to/from different Storage
implementations, to allow experimentally enabling (and if
necessary, disabling) SQLiteStorage. All that's left to do
is the final wiring to turn it on

PiperOrigin-RevId: 232304458

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

@@ -157,6 +157,7 @@ public final class SimpleCache implements Cache {
     } catch (CacheException e) {
       Log.e(TAG, "Storing index file failed", e);
     } finally {
+      index.release();
       unlockFolder(cacheDir);
       released = true;
     }

library/core/src/main/java/com/google/android/exoplayer2/util/AtomicFile.java

@@ -52,6 +52,11 @@ public final class AtomicFile {
     backupName = new File(baseName.getPath() + ".bak");
   }
 
+  /** Whether the file or its backup exists. */
+  public boolean exists() {
+    return baseName.exists() || backupName.exists();
+  }
+
   /** Delete the atomic file. This deletes both the base and backup files. */
   public void delete() {
     baseName.delete();