* Support RN 0.55.4

This commit adds support for RN 0.55.4 by introducing version specific build flavors - reactNative51

* Update scripts after introducing build flavors

* Update Android installation instruction on specifying build flavor

* add option to disable tabbar icon tints and add custom selected icons

*  update docs for disabling tabbar icon tint and selectedIcon

* move new options to ios-specific section in docs

* delete trailing comma in docs

* Screen background option fixed in iOS

* fix unit tests

* add addicional options to tabbar

* fix tests

* [ios11+] Add searchBar option for topBar

* Fix formatting and linter issues

* Add docs about topBar.searchBar

* Fix missing semicolons

* Revert prettier changes

* Add js tests for onSearchBarUpdated to achieve 100% coverage

* Mark searchBar test as :ios: specific

This reverts commit 685009bc23.

Related to #2892

To use it, Override `createReactNativeHost` in MainApplication as follows:

```java
    @Override
    protected ReactNativeHost createReactNativeHost() {
        return new NavigationReactNativeHost(this) {
            @Override
            protected UIImplementationProvider getUIImplementationProvider() {
                return new SyncUiImplementation.Provider();
            }
        };
    }
```

This commit also changes the way TopBar background color is applied.
Instead of applying it to the TitleBar, it's applied to the entire TopBar layout

This option allows to control when tab titles are displayed

* showWhenActive: when a tab is focused
* alwaysShow: show regardless of which tab is in
* alwaysHide: never show tab titles

This commit also changes the way TopTabs options are handled. As the TabLayout
is part of the TopBar, it's the StackControllers responsibility to set TopTabs options.

Hopefully increasing button size will eliminate flakyness

* V2: Peek and pop

* Add event when preview is dismissed in non commit mode

* tslint

* Refactor playground. Document preview api

* Added width prop

* Implement hybrid push/preview button

* Fixing small linting errors

* Testing new multi pr job

* Disable unmounts when dismissed

This works around an issue with RN's view reconciliation when the
root component has no background color defined.