feat(subtitles): optionally anchor text subtitles to the screen bottom

Adds an "Anchor to Screen" toggle under Subtitle Styling (Android +
ExoPlayer only, default off). When enabled, the text SubtitleView is
sized to the full container instead of the letterboxed video rect, so
SRT/VTT/mov_text cues render in the black bars below widescreen video
and font size and the position setting become relative to the physical
screen height. Bitmap (PGS/VOB) and ASS/libass rendering are unchanged;
mpv already places plaintext subtitles in the margins by default.

close #1730

test/services/music/music_playback_service_test.dart

@@ -368,6 +368,7 @@ class FakePlayer implements Player {
     int subtitlePosition = 100,
     bool bold = false,
     bool italic = false,
+    bool anchorToScreen = false,
   }) async {}
 
   @override