Show back button in status bar when in an activity in front of the lockscreen.

When the lockscreen is up we still enable the home and recent app buttons,
but the back button is only enabled if the lockscreen is not obscured by other activities

Fixes problem escaping from the DeskClock app when the screen is locked.

Bug: 5206013

Change-Id: I68c9e4513018f51e2ee4fc11254e42ecbef834e4
Signed-off-by: Mike Lockwood <lockwood@android.com>