Properly resume paused activity when docked stack in maximized

Force focus to the fullscreen stack so that its activity is resumed
regardless of it current state when moving all tasks from the docked
stack to the fullscreen stack and the fullscreen stack should be on
top.

Bug: 28247643
Bug: 28196862
Change-Id: Ic60c3794db765e9ee31b0facbbc4f461d65f30c2