This commit contains various fixes and enhancements, mostly because I
was too lazy to commit small changes.

* Add topBarCollapseOnScroll style property which compliments
  navBarHideOnScroll. This property is needed when showing a
  collapsing topTabs screen. In this case the developer might to keep
  the titleBar bar when the screen is collapsed.
* Reduce collapse sensitivity by taking scaled touch slop into account
* Cancel touches when collapsing. This fix prevents buttons from
  responding to touch events when collapse ends.

Add expendOnTopTabChange style property which defaults to true. Setting it to false

* Less intrusive collapsing implementation

In order for a screen to implement collapsingTopBar, it had to define
a specific set of style properties such as drawUnderTabBar or
drawUnderNavBar. This commit aims to make implementing collapsingTopBar
less intrusive by inferring these properties from hideTitleBarOnScroll.

This commit introduces a few fixes and changes related to
the collapsingTopBar mechanism.

1. Screens are now drawn under bottomTabs if titleBarHideOnScroll
   is true.

2. TitleBarHideOnScroll can be passed in AppStyle.

3. ScrollView is detected properly after it gets detached due to large
   diff in render.

4. Take BottomTabs height into account when measuring collapsing
   screens.

* Implement screenBackgroundColor

* Collapse ViewPagerScreen on TopTab select

* Only destroy CollapsingContentView

* Fix Fab

Implement collapsing TopTabs on Android

specify `navBarHideOnScroll: true` in your `TopTabsScreen`