Fix navbar window fitting on Android. (#235)

This change fixes the issue of handling transparent status bar. In such a case the navbar should became slightly bigger in order to fill the status bar space that now belongs to the window. In order for that to happen we use `setFitsSystemWindows` on main screen layout, on app bar layout and on toolbar (the toolbar is the view that gets bigger in the end).

Note that this currently only work if we use Android's native mechanism for changingthe status bar behavior and not using `StatusBar` module from RN. It is because `StausBar` module actually strips top insets from the event that gets delivered to the views. For the time being you can try the following to change status bar:
```
getWindow().getDecorView().setSystemUiVisibility(View.SYSTEM_UI_FLAG_LAYOUT_STABLE | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN);
```

Note that we are also disabling app bar's scroll behavior. We wasn't utilising this yet, and it cause some weird errors on hot reload because scrolling behavior actually sets `firsSystemWindows` flag on the screen view which ended up consuming insets before appbar would get them.

android/src/main/java/com/swmansion/rnscreens/ScreenStackHeaderConfig.java

@@ -47,6 +47,7 @@ public class ScreenStackHeaderConfig extends ViewGroup {
     setVisibility(View.GONE);
 
     mToolbar = new Toolbar(context);
+    mToolbar.setFitsSystemWindows(true);
 
     // set primary color as background by default
     TypedValue tv = new TypedValue();