Recover from empty timelines.

(Related to GitHub #1706)

When the timeline becomes empty, the playback state transitions to "ended".
When the timeline becomes non-empty again, exceptions are thrown because MSG_DO_SOME_WORK is still regularly sent and media periods are getting prepared.

This change ensures that no MSG_DO_SOME_WORK messages are sent in "ended" state.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=160537147

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

@@ -618,7 +618,7 @@ import java.io.IOException;
 
     if ((playWhenReady && state == ExoPlayer.STATE_READY) || state == ExoPlayer.STATE_BUFFERING) {
       scheduleNextWork(operationStartTimeMs, RENDERING_INTERVAL_MS);
-    } else if (enabledRenderers.length != 0) {
+    } else if (enabledRenderers.length != 0 && state != ExoPlayer.STATE_ENDED) {
       scheduleNextWork(operationStartTimeMs, IDLE_INTERVAL_MS);
     } else {
       handler.removeMessages(MSG_DO_SOME_WORK);
@@ -831,7 +831,7 @@ import java.io.IOException;
       for (ExoPlayerMessage message : messages) {
         message.target.handleMessage(message.messageType, message.message);
       }
-      if (mediaSource != null) {
+      if (state == ExoPlayer.STATE_READY || state == ExoPlayer.STATE_BUFFERING) {
         // The message may have caused something to change that now requires us to do work.
         handler.sendEmptyMessage(MSG_DO_SOME_WORK);
       }