Move taskbar to be closer to QSB

- Offset taskbar to be halfway between QSB and bottom of the screen.
- Add taskbar translationY state property to animate when going
  between launcher and an app.
- Draw the taskbar background in TaskbarContainerView instead of
  TaskbarView so it stays pinned to the bottom as TaskbarView
  translates up. If we want to have a background behind the
  taskbar on home, this should move back to TaskbarView so that
  the drawing can be shared by mTaskbarViewOnHome.

Test: visually on home screen, translates when entering and
exiting an app

Bug: 182981881
Bug: 171917176
Change-Id: I44f8b2c770074f7f015dcccbc2befd3453811193