When ever bundle was reloaded, all currently registered screens
where recreated since they were still attached to the react context.
This commit adds, in a very hacky way, means to respond to bundle
reload events.

Since Toolbar icons' height is set according to Toolbar height,
We need to ensure the call to `setupToolbar` from `onCreate` is
called after the Toolbar is measured.

* Due to race condition when showing modal, buttons were added to
previous Toolbar.

* Minor refactor, moved `updateStyles` from BaseReactActivity to
StyleHelper class since it's now also used by RnnModal

Toolbar buttons not updating
Pushes into BottomTabActivity not always working
Modal not closing properly when back pressed

Minor refactor after merging #52.

This commit also fixes issue where style was not cleared when changing
tabs, and when pushing or popping screens.

Navigation views such as ToolBar, StatusBar, NavigationBar can now be
customized through a static JSON defined in each screen component.

Styling options introduced in this commit:

 - ToolBar color
 - StatusBar color
 - NavigationBar color
 - Buttons tint color
 - Title color
 - Tab normal text color
 - Tab Selected text color
 - Selected tab inidicator color

Navigator.push allows us to open a new screen. Currently it starts a new Activity with the screen
as it’s root view, but this is only a temporary behaviour. The intended behaviour is similar to
View Controllers on iOS which allows you to manage view hierarchies or a stack of fragments
managed by FragmentManager on Android.