Set focused activity when moving task fullscreen.

When maximizing docked stack & docked activity is not resumed & focused
we need to not only move stack to the top, but also mark the activity
as the focused one. Otherwise the activity that is getting hidden (since
its stack is going under fullscreen stack) will retain focus.

Bug: 24870602
Change-Id: Ie929fc95ba7aaccd67497ae6ede32e64dc1ba094