Add TaskbarView as a child of Hotseat

Instead of keeping the Taskbar window showing above the home screen,
we now integrate a copy of TaskbarView directly into Launcher's view
hierarchy. Most TaskbarController calls apply to both TaskbarViews
(mTaskbarViewInApp and mTaskbarViewOnHome), except for calls related
to Hotseat, which only apply to mTasbkarViewInApp given the real
Hotseat will be showing with mTaskbarViewOnHome.

More cleanup will follow this change.

Test: All taskbar interactions (e.g. drag and drop) continue to work
Bug: 182512211
Bug: 171917176
Change-Id: I0f0b124f652daa85f866e1df8f9e2981540331a0