Set BottomTabs background color only if it is defined

This change means that in a tab based app, when pushing a screen
which did not declare `tabBarBackgroundColor` - the BottomTabs color
will persist from the previous screen and not be set to the default
color which is white. This eliminates the unwanted BottomTabs
background color change in this specific use case.

closes #409

android/app/src/main/java/com/reactnativenavigation/layouts/BottomTabsLayout.java

     }
 
     private class ScreenStackNotFoundException extends RuntimeException {
-        public ScreenStackNotFoundException(String navigatorId) {
+        ScreenStackNotFoundException(String navigatorId) {
             super(navigatorId);
         }
     }

android/app/src/main/java/com/reactnativenavigation/params/StyleParams.java

 
     public boolean drawScreenAboveBottomTabs;
 
-    public Color snackbarTextColor;
     public Color snackbarButtonColor;
 
     public boolean bottomTabsHidden;

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

     }
 
     public void setStyleFromScreen(StyleParams params) {
-        setBackgroundColor(params.bottomTabsColor);
-
+        if (params.bottomTabsColor.hasColor()) {
+            setBackgroundColor(params.bottomTabsColor);
+        }
         if (params.bottomTabsButtonColor.hasColor()) {
             setInactiveColor(params.bottomTabsButtonColor.getColor());
         }
-
         if (params.selectedBottomTabsButtonColor.hasColor()) {
             setAccentColor(params.selectedBottomTabsButtonColor.getColor());
         }