Hide nav bar when keyguard on the 2nd display

Bug: 130963707
Test: atest SystemUITest
Test: atest DisplayPolicyLayoutTests#layoutWindowLw_keyguardDialog_hideNav
Test: manual - use chromecast with debug patch + force Desktop mode
               Press power key to make the lockscreen show and check if
	       nav bar is gone
Change-Id: I7f8f9c2c6d1157218e5dda1068170799fdccb64b