[DO NOT MERGE] User switcher activity - use immersive mode

Use insetscontroller to enter immersive mode, and hide the systembars,
per spec.

Test: manual - use activity, go to overview, go back, show/hide
taskbar
Fixes: 223537157

Change-Id: I40bba7f495d3983870df3abe8d2abbed1c52c81d