Add buffer size based adaptive track selection.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=222221294

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

@@ -197,9 +197,10 @@ public class DefaultLoadControl implements LoadControl {
 
     /** Creates a {@link DefaultLoadControl}. */
     public DefaultLoadControl createDefaultLoadControl() {
+      Assertions.checkState(!createDefaultLoadControlCalled);
       createDefaultLoadControlCalled = true;
       if (allocator == null) {
-        allocator = new DefaultAllocator(true, C.DEFAULT_BUFFER_SEGMENT_SIZE);
+        allocator = new DefaultAllocator(/* trimOnReset= */ true, C.DEFAULT_BUFFER_SEGMENT_SIZE);
       }
       return new DefaultLoadControl(
           allocator,

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

@@ -774,6 +774,7 @@ import java.util.concurrent.atomic.AtomicBoolean;
     for (Renderer renderer : enabledRenderers) {
       renderer.resetPosition(rendererPositionUs);
     }
+    notifyTrackSelectionDiscontinuity();
   }
 
   private void setPlaybackParametersInternal(PlaybackParameters playbackParameters) {
@@ -1172,6 +1173,22 @@ import java.util.concurrent.atomic.AtomicBoolean;
     }
   }
 
+  private void notifyTrackSelectionDiscontinuity() {
+    MediaPeriodHolder periodHolder = queue.getFrontPeriod();
+    while (periodHolder != null) {
+      TrackSelectorResult trackSelectorResult = periodHolder.getTrackSelectorResult();
+      if (trackSelectorResult != null) {
+        TrackSelection[] trackSelections = trackSelectorResult.selections.getAll();
+        for (TrackSelection trackSelection : trackSelections) {
+          if (trackSelection != null) {
+            trackSelection.onDiscontinuity();
+          }
+        }
+      }
+      periodHolder = periodHolder.getNext();
+    }
+  }
+
   private boolean shouldTransitionToReadyState(boolean renderersReadyOrEnded) {
     if (enabledRenderers.length == 0) {
       // If there are no enabled renderers, determine whether we're ready based on the timeline.

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

@@ -155,6 +155,13 @@ public interface TrackSelection {
   void onPlaybackSpeed(float speed);
 
   /**
+   * Called to notify the selection of a position discontinuity.
+   *
+   * <p>This happens when the playback position jumps, e.g., as a result of a seek being performed.
+   */
+  default void onDiscontinuity() {}
+
+  /**
    * @deprecated Use and implement {@link #updateSelectedTrack(long, long, long, List,
    *     MediaChunkIterator[])} instead.
    */