Add layout_gravity option

When layout has no `android:layout_gravity` attribute or set it to `"bottom"`, it is exactly same with past AndroidSlidingUpPanel d38270f531d77d8f2bffe3b8baf1d08b5acdb83a.
If `android:layout_gravity` set to `"top"`, the slideable view goes to the top of layout and we can drag it down to expand.