If ReactContext is null, the event is emitted when onReactContextInitialized is invoked. Closes #3392

* Constants support

* Change Navigation.constants() to an async function on Android

Also added topBarHeight and bottomTabsHeight

I was unable to compile because I use multidex and this line of code force the multidex library to be version 25.4.0 when the latest for this one is 1.0.3. This little fix may save a lot headaches.

title was renamed to text

We need handle situation where currently emitAppLaunched is being called with null context.
Related to #3392

* Refactored bottomTab and bottomTabs

* Initial commit before switching to our fork of AHBottomNavigation

* Bump AHBottomNavigation version

* More work on BottomTab options

Need to merge some child options when a child is attached to parent

* Fix tests

* DefaultOptions refactor

* defaultOptions are now merged before applying options
* Handle BottomTab options in dedicated presenter
* Create BottomTabs in BottomTabsController.createView() instead of constructor

* Set bottomTab TypeFace

* Update Android installation guide

* fixed unit tests

* Fix unit tests

Kind of a workaround

* Handle NPE in getPreferredHeight

If a view hasn't been attached yet, return 0 height

* Apply BottomTab text and icon color individually

wix-playground/AHBottomNavigation does not support setting color for the entire tab (icon and text),
instead it supports setting color individually to text and icon. Another change is that it no longer supports
default color for icons. If icon color is undefined - it won't tint the icon.

* Add getCurrentChild to ParentController contract

* Clear backButton when setting stack root

* Add  ViewController.resolveCurrentOptions

resolveCurrentOptions merges the controllers options with all of its current children

* StackController refactor

* Create pushed child view only in StackController.createView
* When initialising a stack with multiple children, only create the top child's view
* StackController constructor now accepts children
* When pushing child to stack, apply options that change LayoutParams

* Apply BottomTabs layout options when tab views are created

* bototmTab changes in iOS

* options fix

* Fixes options propagating

Closes #3414
Closes #3412

Summary: This adds a new event `onSearchBarCancelPressed` that fires
when the user presses the cancel button on the search bar in the
navigation bar. `onSearchBarUpdated` does indirectly fire when the user
presses cancel, as the query will be reset to an empty string, but it is
not possible to distinguish those events from the user simply resetting
the query. My use case for this is to have a view that appears on top of
the normal content of the view controller when the search bar is
focused.

Test Plan: ran `npm run test-js` and confirmed added JS unit tests
passed.

I also tested this with my app, and confirmed the
`onSearchBarCancelPressed` method was called when I pressed the cancel
button.

* test

* Added jest coverage reporter html for Jenkins

RNN's ReactNativeHost implementation prevented users from adding MainReactPackage
themselves, this forced users that wanted to use a custom MainReactPackage to extend
NavigationReactNativeHost which is a bit of an overkill for this simple use case.

closes #3387

There was an issue where ReactRootView was not unmounted if it was destroyed when it was hidden.

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.