Add abstract superclass for Crypto resources

Necessary for implementing scheme-independant resource management.

Example: A media source needs to have ownership of a crypto resource
while samples are in buffer (but not yet in the renderer).
PiperOrigin-RevId: 241698575

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

+/*
+ * Copyright (C) 2019 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;
+
+/**
+ * A reference-counted resource used in the decryption of media samples.
+ *
+ * @param <T> The reference type with which to make {@link Owner#onLastReferenceReleased} calls.
+ */
+public abstract class CryptoResource<T extends CryptoResource<T>> {
+
+  /**
+   * Implemented by the class in charge of managing a {@link CryptoResource resource's} lifecycle.
+   */
+  public interface Owner<T extends CryptoResource<T>> {
+
+    /**
+     * Called when the last reference to a {@link CryptoResource} is {@link #releaseReference()
+     * released}.
+     */
+    void onLastReferenceReleased(CryptoResource<T> resource);
+  }
+
+  // TODO: Consider adding a handler on which the owner should be called.
+  private final CryptoResource.Owner<T> owner;
+  private int referenceCount;
+
+  /**
+   * Creates a new instance with no incoming references.
+   *
+   * @param owner The owner of this instance.
+   */
+  public CryptoResource(Owner<T> owner) {
+    this.owner = owner;
+    referenceCount = 0;
+  }
+
+  /** Increases by one the incoming reference count for this resource. */
+  public void acquireReference() {
+    referenceCount++;
+  }
+
+  /**
+   * Decreases by one the incoming reference count for this resource, and notifies the owner if said
+   * count reached zero as a result of this operation.
+   *
+   * <p>Must only be called as releasing counter-part of {@link #acquireReference()}.
+   */
+  @SuppressWarnings("unchecked")
+  public synchronized void releaseReference() {
+    if (--referenceCount == 0) {
+      owner.onLastReferenceReleased(this);
+    } else if (referenceCount < 0) {
+      throw new IllegalStateException("Illegal release of resource.");
+    }
+  }
+}