Take margin top into account for slidable view

Created by: thuytrinh

First, thanks for the awesome lib!

How? It'd be nice if the slidable view can be aware of its margin top so that we can achieve the following effect.

There're many different ways to make it work but the simplest we found is to specify layout_marginTop for the slidable view with whatever dimen we expect:

<!-- SLIDING LAYOUT -->
<LinearLayout
    android:layout_marginTop="?actionBarSize"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:clickable="true"
    android:focusable="false"
    android:id="@+id/dragView">

Currently the slidable view always matches its parent (SlidingUpPanelLayout) height.