CEA608: PAINT-ON Mode must keep the last shown captions on the screen

[Problem]
PAINT-ON mode is not implemented. From the compliance tests:
* RDC command has no effect except to select paint-on style.
* Next data are written directly to the display upon receipt.
* If other captioning is already on the screen, the four-row
limit is still in effect.

[Solution]
It is a rare use case, we do not support overriding characters in
existing cueBuilders as PAINT-ON would require. But several
compliance tests check if the screen is cleared when the mode
switch happens. We must keep the old captions when switching
to PAINT-ON mode

[Test]
- Live Over-the-Air content, beginning of commercials often uses
PAINT-ON mode

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

@@ -559,8 +559,19 @@ private void setCaptionMode(int captionMode) {
     int oldCaptionMode = this.captionMode;
     this.captionMode = captionMode;
 
-    // Clear the working memory.
+    // Clear the cues and cueBuilders except for Paint-on mode. Paint-on mode may modify characters
+    // already on the screen. This feature is not fully supported, but we need to keep the previous
+    // screen content shown
+    if (captionMode == CC_MODE_PAINT_ON) {
+      // update the Mode member of all existing cueBuilders even if we are mid-row
+      for (CueBuilder builder : cueBuilders) {
+        builder.setCaptionMode(captionMode);
+      }
+      return;
+    }
+
     resetCueBuilders();
+
     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.
@@ -653,6 +664,10 @@ public CueBuilder(int captionMode, int captionRowCount) {
       setCaptionRowCount(captionRowCount);
     }
 
+    public void setCaptionMode(int captionMode) {
+      this.captionMode = captionMode;
+    }
+
     public void reset(int captionMode) {
       this.captionMode = captionMode;
       cueStyles.clear();