Fix subtitle outline width not applying in release builds (#1687)

## Description
Fix applying the subtitle outline width in ExoPlayer for release builds

The outline width field isn't public, so reflection is used to bypass
and modify it. Since proguard/r8 renames all of the classes and fields,
this meant the field was no longer found. The fix is to have proguard
preserve the necessary field names.

### Related issues
Fixes #1681

### Testing
Emulator w/ release build

## Screenshots
N/a

## AI or LLM usage
None

app/proguard-rules.pro

@@ -11,3 +11,14 @@
 
 # TODO investigate using smaller scope
 -keep class com.google.common.cache.** { *; }
+
+# Media3SubtitleOverride uses reflection to access/modify these fields
+-keep class androidx.media3.ui.SubtitleView {
+  private androidx.media3.ui.SubtitleView$Output output;
+}
+-keep class androidx.media3.ui.CanvasSubtitleOutput {
+  private final java.util.List painters;
+}
+-keep class androidx.media3.ui.SubtitlePainter {
+  private final float outlineWidth;
+}