Voice message scrubbing improvements (#1847)

- Voice messages can be scrubbed (i.e. seeked to) even when they have not been played yet..
- The progress bar is displayed also when paused.
- Multiple voice messages can keep their state when paused.
- Tries to adhere as much as possible at the detailed "green cursor" behavior in the story (but might not be 100% compliant).

Story: https://github.com/vector-im/element-meta/issues/2113

libraries/mediaplayer/api/src/main/kotlin/io/element/android/libraries/mediaplayer/api/MediaPlayer.kt

@@ -38,6 +38,7 @@ interface MediaPlayer : AutoCloseable {
         uri: String,
         mediaId: String,
         mimeType: String,
+        startPositionMs: Long = 0,
     ): State
 
     /**
@@ -69,6 +70,10 @@ interface MediaPlayer : AutoCloseable {
          * Whether the player is currently playing.
          */
         val isPlaying: Boolean,
+        /**
+         * Whether the player has reached the end of the current media.
+         */
+        val isEnded: Boolean,
         /**
          * The id of the media which is currently playing.
          *