Don't consider switching tracks as "joining".

I'm not really sure how best to document this in TrackRenderer;
it's a bit of a weird feature. For now, I've gone with the vague
approach.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=112150939

library/src/main/java/com/google/android/exoplayer/ExoPlayerImplInternal.java

@@ -640,7 +640,9 @@ import java.util.concurrent.atomic.AtomicInteger;
     if (shouldEnable) {
       // Re-enable the renderer with the newly selected track.
       boolean playing = playWhenReady && state == ExoPlayer.STATE_READY;
-      renderer.enable(trackIndex, positionUs, playing);
+      // Consider as joining if the renderer was previously disabled, but not when switching tracks.
+      boolean joining = !isEnabled && playing;
+      renderer.enable(trackIndex, positionUs, joining);
       enabledRenderers.add(renderer);
       if (playing) {
         renderer.start();

library/src/main/java/com/google/android/exoplayer/TrackRenderer.java

@@ -150,9 +150,7 @@ public abstract class TrackRenderer implements ExoPlayerComponent {
    *
    * @param track The track for which the renderer is being enabled.
    * @param positionUs The player's current position.
-   * @param joining Whether this renderer is being enabled to join an ongoing playback. If true
-   *     then {@link #start} must be called immediately after this method returns (unless a
-   *     {@link ExoPlaybackException} is thrown).
+   * @param joining Whether this renderer is being enabled to join an ongoing playback.
    * @throws ExoPlaybackException If an error occurs.
    */
   /* package */ final void enable(int track, long positionUs, boolean joining)
@@ -169,9 +167,7 @@ public abstract class TrackRenderer implements ExoPlayerComponent {
    *
    * @param track The track for which the renderer is being enabled.
    * @param positionUs The player's current position.
-   * @param joining Whether this renderer is being enabled to join an ongoing playback. If true
-   *     then {@link #onStarted} is guaranteed to be called immediately after this method returns
-   *     (unless a {@link ExoPlaybackException} is thrown).
+   * @param joining Whether this renderer is being enabled to join an ongoing playback.
    * @throws ExoPlaybackException If an error occurs.
    */
   protected void onEnabled(int track, long positionUs, boolean joining)