MPV: Adjust subtitle delay/offset (#470)

**This is for MPV playback backend only!** Since this only works in MPV
for now, the option is not shown if using ExoPlayer.

Adds a setting (the gear icon during playback) to adjust the subtitle
delay.

A negative delay shows subtitles sooner. A positive delay shows them
later. For example, if a subtitle is supposed to show between
1000ms-2000ms, a negative 250ms delay will show it at 750ms-1750ms
instead.

The delay is persisted between plays of the same media & subtitle track.

Ref: #12 


![subtitle_delay](https://github.com/user-attachments/assets/824e3a28-650a-4619-9535-b67addaabf1e)

app/src/main/res/values/strings.xml

@@ -393,6 +393,7 @@
     <string name="favorite_channels_at_beginning">Show favorite channels first</string>
     <string name="sort_channels_recently_watched">Sort channels by recently watched</string>
     <string name="color_code_programs">Color-code programs</string>
+    <string name="subtitle_delay">Subtitle delay</string>
 
     <string-array name="theme_song_volume">
         <item>Disabled</item>