Change BottomTabs visibility to Invisible instead of Gone

This change is introduced so BottomTabs are actually rendered when created
This commit also fixes back button issue where it wasn't properly added when pushing screens.

lib/android/app/src/main/java/com/reactnativenavigation/parse/BackButton.java

         if (!enabled.hasValue()) enabled = defaultOptions.enabled;
         if (!testId.hasValue()) testId = defaultOptions.testId;
     }
+
+    public void setVisible() {
+        visible = new Bool(true);
+        hasValue = true;
+    }
 }

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/bottomtabs/BottomTabsController.java

     private void attachTabs() {
         for (int i = (tabs.size() - 1); i >= 0; i--) {
             ViewGroup tab = tabs.get(i).getView();
-            if (i != 0) tab.setVisibility(View.GONE);
+            if (i != 0) tab.setVisibility(View.INVISIBLE);
             getView().addView(tab);
         }
     }
 
     @Override
     public void selectTab(final int newIndex) {
-        getCurrentView().setVisibility(View.GONE);
+        getCurrentView().setVisibility(View.INVISIBLE);
         bottomTabs.setCurrentItem(newIndex, false);
         getCurrentView().setVisibility(View.VISIBLE);
     }

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/stack/BackButtonHelper.java

 package com.reactnativenavigation.viewcontrollers.stack;
 
 import com.reactnativenavigation.parse.Options;
-import com.reactnativenavigation.parse.params.Bool;
 import com.reactnativenavigation.viewcontrollers.ViewController;
 
 public class BackButtonHelper {
     public void addToChild(StackController stack, ViewController child) {
         if (stack.size() <= 1 || child.options.topBar.buttons.left != null || child.options.topBar.buttons.back.visible.isFalse()) return;
         Options options = new Options();
-        options.topBar.buttons.back.visible = new Bool(true);
+        options.topBar.buttons.back.setVisible();
         child.mergeOptions(options);
     }
 }

lib/android/app/src/test/java/com/reactnativenavigation/viewcontrollers/BottomTabsControllerTest.java

     public void setTabs_firstChildIsVisibleOtherAreGone() {
         uut.onViewAppeared();
         for (int i = 0; i < uut.getChildControllers().size(); i++) {
-            assertThat(uut.getView().getChildAt(i).getVisibility()).isEqualTo(i == 0 ? View.VISIBLE : View.GONE);
+            assertThat(uut.getView().getChildAt(i).getVisibility()).isEqualTo(i == 0 ? View.VISIBLE : View.INVISIBLE);
         }
     }
 
         uut.onTabSelected(3, false);
 
         assertThat(uut.getSelectedIndex()).isEqualTo(3);
-        assertThat(((ViewController) ((List) uut.getChildControllers()).get(0)).getView().getVisibility()).isEqualTo(View.GONE);
+        assertThat(((ViewController) ((List) uut.getChildControllers()).get(0)).getView().getVisibility()).isEqualTo(View.INVISIBLE);
         assertThat(((ViewController) ((List) uut.getChildControllers()).get(3)).getView().getVisibility()).isEqualTo(View.VISIBLE);
         verify(eventEmitter, times(1)).emitBottomTabSelected(0, 3);
     }