HDR/Dolby Vision playback on Fire TV (and some Sony/Philips models)
shows gray letterbox bars instead of black on OLED panels: the bars
were the player container's window-plane background, and those device
compositors raise SDR graphics-plane black while the display is in
HDR/DV output mode. Black inside the video plane is unaffected (same
mechanism as ExoPlayer #8803 and Kodi #25300).
Add a fullscreen, buffer-less SurfaceView beneath the video surface in
the shared player container. Like any below-window SurfaceView it
registers its rect as a window transparent region, so the letterbox
area scans out as the SurfaceFlinger backdrop instead of window-plane
pixels. No buffer is ever posted: SurfaceFlinger skips buffer-less
layers, and drawing into one would put the bars back on an SDR layer.
Covers both the ExoPlayer and mpv backends.
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