Add LocalMediaDrmCallback. Useful for providing local keys.

Issue: #3178

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=166330215

library/core/src/main/java/com/google/android/exoplayer2/drm/LocalMediaDrmCallback.java

+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.android.exoplayer2.drm;
+
+import com.google.android.exoplayer2.drm.ExoMediaDrm.KeyRequest;
+import com.google.android.exoplayer2.drm.ExoMediaDrm.ProvisionRequest;
+import com.google.android.exoplayer2.util.Assertions;
+import java.io.IOException;
+import java.util.UUID;
+
+/**
+ * A {@link MediaDrmCallback} that provides a fixed response to key requests. Provisioning is not
+ * supported. This implementation is primarily useful for providing locally stored keys to decrypt
+ * ClearKey protected content. It is not suitable for use with Widevine or PlayReady protected
+ * content.
+ */
+public final class LocalMediaDrmCallback implements MediaDrmCallback {
+
+  private final byte[] keyResponse;
+
+  /**
+   * @param keyResponse The fixed response for all key requests.
+   */
+  public LocalMediaDrmCallback(byte[] keyResponse) {
+    this.keyResponse = Assertions.checkNotNull(keyResponse);
+  }
+
+  @Override
+  public byte[] executeProvisionRequest(UUID uuid, ProvisionRequest request) throws IOException {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public byte[] executeKeyRequest(UUID uuid, KeyRequest request) throws Exception {
+    return keyResponse;
+  }
+
+}