Fixing several bugs.

- Prevent long pressing to drag when a task is already docked
- Ensure that the history button has a bit of background protection
- When fast-toggling, the second tap should actually activate the 
  second task and not the third
- Align the history view with the stack view
- Fixing regression launching affiliated tasks

Bug: 25931916

Change-Id: I9d74d0d2427ea0b11f4817d0e635013065e3a792