fix(core): make frame pts and dts immutable

Author: ThibaultBee

core/src/main/java/io/github/thibaultbee/streampack/core/elements/data/Frame.kt

@@ -61,12 +61,12 @@ data class Frame(
     /**
      * Presentation timestamp in µs
      */
-    var ptsInUs: Long,
+    val ptsInUs: Long,
 
     /**
      * Decoded timestamp in µs (not used).
      */
-    var dtsInUs: Long?,
+    val dtsInUs: Long? = null,
 
     /**
      * [Boolean.true] if frame is a key frame (I-frame for AVC/HEVC and audio frames)