fix(android): subtitles on the picture, not on a black bar

Every subtitle line arrived in an opaque black box wide enough to sit across
the picture. That box is what Android hands back when the viewer has set no
captioning preferences: SubtitleView.setUserDefaultStyle() reads the system
style and falls back to media3's DEFAULT, which is white on opaque black.

Dropping the box is not the same as replacing the style. Someone who has
configured captions in accessibility settings has said something specific about
colour, typeface and edges, and overriding all of it to remove a background
would answer a question they did not ask. Their style is kept and only the two
colours that paint a box — background and window — are cleared.

A style specifying no edge gets a black outline, since without a box the text
must supply its own contrast or it is unreadable over a bright scene. One that
already names an edge keeps it: that viewer has said how they want their
captions separated from the picture.

Compiles and packages, but NOT yet seen on a device — the tablet was
disconnected before it could be deployed, so the requirement is recorded as
"Done (pending device verification)" and this must not be tagged into a release
until someone has looked at it.

TRACES: UR-020 | DR-261

CHANGELOG.md

@@ -9,6 +9,17 @@ generated trace matrix lives in [docs/traceability.md](docs/traceability.md).
 For how long each fixed defect had been shipping before it was found, see
 [docs/defect-windows.md](docs/defect-windows.md).
 
+## Unreleased
+
+### 🐛 Fixes
+
+- **Subtitles sit on the picture instead of on a black bar.** Every line arrived
+  in an opaque box, which is what Android hands back when no captioning
+  preferences have been set. The box is gone; the text keeps its own outline so
+  it stays readable over a bright scene. If you *have* set up captions in
+  Android's accessibility settings, your colours, typeface and edges are still
+  used — only the background is cleared. (UR-020 → DR-261)
+
 ## v0.11.2
 
 ### 🐛 Fixes