Fix left position for subtitles.

When SubtitlePainter positions the cues centred in the given box, it must add
the left offset of the box to get the correct position.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=215535289

library/ui/src/main/java/com/google/android/exoplayer2/ui/SubtitlePainter.java

@@ -310,7 +310,7 @@ import com.google.android.exoplayer2.util.Util;
       textLeft = Math.max(textLeft, parentLeft);
       textRight = Math.min(textLeft + textWidth, parentRight);
     } else {
-      textLeft = (parentWidth - textWidth) / 2;
+      textLeft = (parentWidth - textWidth) / 2 + parentLeft;
       textRight = textLeft + textWidth;
     }