Support hiding back button in Toolbar

android/app/src/main/java/com/reactnativenavigation/core/objects/Screen.java

     private static final String KEY_NAVIGATION_BAR_COLOR = "navigationBarColor";
     private static final String KEY_NAV_BAR_BUTTON_COLOR = "navBarButtonColor";
     private static final String KEY_NAV_BAR_TEXT_COLOR = "navBarTextColor";
+    private static final String KEY_BACK_BUTTON_HIDDEN = "backButtonHidden";
     private static final String KEY_TAB_NORMAL_TEXT_COLOR = "tabNormalTextColor";
     private static final String KEY_TAB_SELECTED_TEXT_COLOR = "tabSelectedTextColor";
     private static final String KEY_TAB_INDICATOR_COLOR = "tabIndicatorColor";
     public final String navigatorEventId;
     public final String icon;
     public ArrayList<Button> buttons;
+    public final boolean backButtonHidden;
     public HashMap<String, Object> passedProps = new HashMap<>();
 
     // Navigation styling
             passedProps = ((ReadableNativeMap) screen.getMap(KEY_PROPS)).toHashMap();
         }
         buttons = getButtons(screen);
+        backButtonHidden = getBoolean(screen, KEY_BACK_BUTTON_HIDDEN);
         setToolbarStyle(screen);
     }
 

android/app/src/main/java/com/reactnativenavigation/views/RnnToolBar.java

         if (hasDrawer && mDrawerIcon == null) {
             navArrow = (DrawerArrowDrawable) this.getNavigationIcon();
         } else {
-            if (isBack) {
+            if (isBack && !screen.backButtonHidden) {
                 navArrow = new DrawerArrowDrawable(ContextProvider.getActivityContext());
             } else if (hasDrawer) {
                 navIcon = mDrawerIcon;