Add an TrackSelector base interface

This will allow Player to move in common without
trackSelector and all its many dependency.

Currently all users of `getTrackSelector` are
downcasting it in `DefaultTrackSelector`, this change
thus does not break them.

Track selection API is intended to be reworked, methods
will be added to the currently empty interface.

#player-to-common

PiperOrigin-RevId: 346159765

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayer.java

@@ -468,6 +468,10 @@ public interface ExoPlayer extends Player {
     }
   }
 
+  @Override
+  @Nullable
+  TrackSelector getTrackSelector();
+
   /** Returns the {@link Looper} associated with the playback thread. */
   Looper getPlaybackLooper();
 

library/core/src/main/java/com/google/android/exoplayer2/Player.java

@@ -34,7 +34,7 @@ import com.google.android.exoplayer2.source.TrackGroupArray;
 import com.google.android.exoplayer2.text.Cue;
 import com.google.android.exoplayer2.text.TextOutput;
 import com.google.android.exoplayer2.trackselection.TrackSelectionArray;
-import com.google.android.exoplayer2.trackselection.TrackSelector;
+import com.google.android.exoplayer2.trackselection.TrackSelectorInterface;
 import com.google.android.exoplayer2.util.MutableFlags;
 import com.google.android.exoplayer2.util.Util;
 import com.google.android.exoplayer2.video.VideoDecoderOutputBufferRenderer;
@@ -1441,7 +1441,7 @@ public interface Player {
    * Returns the track selector that this player uses, or null if track selection is not supported.
    */
   @Nullable
-  TrackSelector getTrackSelector();
+  TrackSelectorInterface getTrackSelector();
 
   /** Returns the available track groups. */
   TrackGroupArray getCurrentTrackGroups();

library/core/src/main/java/com/google/android/exoplayer2/trackselection/TrackSelector.java

@@ -83,7 +83,7 @@ import com.google.android.exoplayer2.util.Assertions;
  * thread. The track selector may call {@link InvalidationListener#onTrackSelectionsInvalidated()}
  * from any thread.
  */
-public abstract class TrackSelector {
+public abstract class TrackSelector implements TrackSelectorInterface {
 
   /**
    * Notified when selections previously made by a {@link TrackSelector} are no longer valid.

library/core/src/main/java/com/google/android/exoplayer2/trackselection/TrackSelectorInterface.java

@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2020 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.trackselection;
+
+import com.google.android.exoplayer2.Player;
+
+/**
+ * The component of a {@link Player} responsible for selecting tracks to be played.
+ *
+ * <p>No Player agnostic track selection is currently supported. Clients should downcast to the
+ * implementation's track selection.
+ */
+// TODO(b/172315872) Define an interface for track selection.
+public interface TrackSelectorInterface {}