SDK / exoplayer

[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