fix(player): make native Android video opt-in again — it shipped as audio with no picture

DR-161 flipped experimentalNativeVideo on by default so picture-in-picture could
shrink a real video surface. On a device that shipped sound with a blank screen.

The decode path was never at fault. Logcat shows ExoPlayer running and feeding a
live SurfaceView with an active BufferQueue. The compositing was: the SurfaceView
sits behind the WebView, and the step that clears the opaque layers above it
never took effect — `WebView transparent = false` is logged, `= true` never
appears. The video was rendering correctly the whole time, behind an opaque page.

This is precisely the defect the flag existed to contain;
VideoPlayer.scrubRegression.test.ts had already recorded that "the native
SurfaceView has never been visible through the webview". Enabling it by default
shipped a verified decode path on top of an unverified display path.

Reverting costs nothing that matters: PiP does not depend on it — DR-160 drives
PiP from the WebView <video> — and working video outranks PiP showing a native
surface. The flag stays in Settings, now described as incomplete rather than as a
performance win, so anyone helping test it still can.

Fixing the compositing is the prerequisite for trying this default again (DR-172).

src/lib/utils/pictureInPicture.ts

@@ -97,9 +97,9 @@ export function setAutoEnterEnabled(enabled: boolean): void {
  * needs for the PiP window's aspect ratio and the play state for its play/pause
  * action.
  *
- * The flag defaults to **on** now (DR-161), so Android normally shrinks the real
- * ExoPlayer surface instead; this remains the path for Linux and for anyone who
- * turned the flag off.
+ * The flag is back to defaulting **off** (DR-172, after native video shipped as
+ * audio with no picture), so this is once again the path Android normally takes —
+ * which is why PiP does not depend on that flag being on.
  *
  * Pass `active: false` when the element goes away, or PiP would be offered over a
  * video that is no longer there.