Fixed potential bug in which old paint-on captions could be drawn overtop by…

Fixed potential bug in which old paint-on captions could be drawn overtop by pop-on captions as they weren't being cleared properly.

This fix was taken from YouTube's Cea608Decoder ([]

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=147066662

library/src/main/java/com/google/android/exoplayer2/text/cea/Cea608Decoder.java

@@ -503,11 +503,14 @@ public final class Cea608Decoder extends CeaDecoder {
       return;
     }
 
+    int oldCaptionMode = this.captionMode;
     this.captionMode = captionMode;
+
     // Clear the working memory.
     resetCueBuilders();
-    if (captionMode == CC_MODE_ROLL_UP || captionMode == CC_MODE_UNKNOWN) {
-      // When switching to roll-up or unknown, we also need to clear the caption.
+    if (oldCaptionMode == CC_MODE_PAINT_ON || captionMode == CC_MODE_ROLL_UP
+        || captionMode == CC_MODE_UNKNOWN) {
+      // When switching from paint-on or to roll-up or unknown, we also need to clear the caption.
       cues = null;
     }
   }