Fix FieldsBelowInit in ReadableMapBuffer.kt (#55405)

Summary:
Pull Request resolved: #55405

Moved offsetForDynamicData property declaration above init block to ensure proper initialization order per Kotlin best practices.

changelog: [internal] internal

Reviewed By: javache

Differential Revision: D91819522

fbshipit-source-id: 69261eab9d4977a790f7c90b823344e82e1d948f

Author: mdvacca

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/common/mapbuffer/ReadableMapBuffer.kt

@@ -39,6 +39,10 @@ private constructor(
   override var count: Int = 0
     private set
 
+  // returns the relative offset of the first byte of dynamic data
+  private val offsetForDynamicData: Int
+    get() = getKeyOffsetForBucketIndex(count)
+
   init {
     readHeader()
   }
@@ -56,10 +60,6 @@ private constructor(
     count = readUnsignedShort(buffer.position()).toInt()
   }
 
-  // returns the relative offset of the first byte of dynamic data
-  private val offsetForDynamicData: Int
-    get() = getKeyOffsetForBucketIndex(count)
-
   /**
    * @param intKey Key to search for
    * @return the "bucket index" for a key or -1 if not found. It uses a binary search algorithm