DefaultDrmSession should handle when key is already available (getKeyRequest() -> REQUEST_TYPE_NONE)

Use case description

When a DRM-protected (e.g. PlayReady) media file is played back for the second time, the DRM key could still be available in the DRM. Then the DRM might return REQUEST_TYPE_NONE on getKeyRequest() (see https://developer.android.com/reference/android/media/MediaDrm.KeyRequest#REQUEST_TYPE_NONE ).
DefaultDrmSession should handle this case.

Proposed solution

The proposed solution is to check the return value of getKeyRequest() and to start playback immediately, when the key is already available. See this patch:

diff --git a/exoplayer/library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSession.java b/exoplayer/library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSession.java
index b357a2ed06..2c93597bdf 100644
--- a/exoplayer/library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSession.java
+++ b/exoplayer/library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSession.java
@@ -20,6 +20,7 @@ import static com.google.android.exoplayer2.util.Assertions.checkStateNotNull;
 import static java.lang.Math.min;
 
 import android.annotation.SuppressLint;
+import android.media.MediaDrm;
 import android.media.NotProvisionedException;
 import android.os.Handler;
 import android.os.HandlerThread;
@@ -500,6 +501,11 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
   private void postKeyRequest(byte[] scope, int type, boolean allowRetry) {
     try {
       currentKeyRequest = mediaDrm.getKeyRequest(scope, schemeDatas, type, keyRequestParameters);
+      if (currentKeyRequest.getRequestType() == KeyRequest.REQUEST_TYPE_NONE) {
+        state = STATE_OPENED_WITH_KEYS;
+        dispatchEvent(DrmSessionEventListener.EventDispatcher::drmKeysLoaded);
+        return;
+      }
       Util.castNonNull(requestHandler)
           .post(MSG_KEYS, Assertions.checkNotNull(currentKeyRequest), allowRetry);
     } catch (Exception e) {

Alternatives considered

The check for REQUEST_TYPE_NONE could be done in requestHandler.post() or below, but then additional changes are necessary; especially some kind of null or empty keyResponse would need to be considered as OK/key-already-available. (IMHO not better.)