Store the resume position even if the window is not seekable

Note: this temporarily exposes a bug where seeking a non-seekable source leads
to the player position being negative before playback starts.

Issue: #2655

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161801111

demo/src/main/java/com/google/android/exoplayer2/demo/PlayerActivity.java

@@ -406,8 +406,7 @@ public class PlayerActivity extends Activity implements OnClickListener, ExoPlay
 
   private void updateResumePosition() {
     resumeWindow = player.getCurrentWindowIndex();
-    resumePosition = player.isCurrentWindowSeekable() ? Math.max(0, player.getContentPosition())
-        : C.TIME_UNSET;
+    resumePosition = Math.max(0, player.getContentPosition());
   }
 
   private void clearResumePosition() {