fix(android): set initialIndex only once (#457)

* Add tabview example

* fix(android): set initialIndex only once

Author: troZee

android/src/main/java/com/reactnativepagerview/NestedScrollableHost.kt

@@ -22,7 +22,7 @@ import kotlin.math.sign
 class NestedScrollableHost : FrameLayout {
   constructor(context: Context) : super(context)
   constructor(context: Context, attrs: AttributeSet?) : super(context, attrs)
-
+  public var initialIndex: Int? = null
   private var touchSlop = 0
   private var initialX = 0f
   private var initialY = 0f

android/src/main/java/com/reactnativepagerview/PagerViewViewManager.kt

@@ -145,8 +145,13 @@ class PagerViewViewManager : ViewGroupManager<NestedScrollableHost>() {
   @ReactProp(name = "initialPage", defaultInt = 0)
   fun setInitialPage(host: NestedScrollableHost, value: Int) {
     val view = getViewPager(host)
-    view.post {
-      setCurrentItem(view, value, false)
+    //https://github.com/callstack/react-native-pager-view/issues/456
+    //Initial index should be set only once. 
+    if (host.initialIndex === null) {
+      view.post {
+        setCurrentItem(view, value, false)
+        host.initialIndex = value
+      }
     }
   }