Merging TopBar title, buttons and status bar options is broken on iOS in 3.1.1.
Parent options aren't resolved properly and default options aren't regarded when they should be.

This commit adds initial support for passing null as a valid value for color properties.
When a color property is applied, if it wasn't specified then the default color is applied. Now, when null is passed, no color will be applied
leaving the last color applied as is.

This is useful when showing an Overlay and you'd like to keep the current StatusBar color unaffected. Another usecase would be when you'd like to avoid coloring an icon in BottomTabs.

* Fix top bar layout frame, Closes #5003

* Set layout child view controllers on initialization

* Remove duplicate initWithLayoutInfo: function

* Stop manually manage viewControllers store, Iterate layout instead

* Update RNNCommandsHandlerTest.m

* Explicitly attach and detach viewControllers in store
* Stop registering components in commands handler