[Quantum compose] move top padding into scrollview

- Move the top padding into the scrollview content so it scrolls
  with as the view fills up.
- Add the click listener for composearea_tap_trap_bottom so
  whenever it is clicked it focuses the body and places the cursor
  at end of the body. It looks like this is what it's supposed to
  do, but I don't know where that code went.

b/17363410

Change-Id: Ib7228cf88c853293602baed3eca45f78d5bddb12
3 files changed