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>