Handle showWhenLocked on secondary displays

The keyguard has windows on the default display and the remote display
selected by MediaRouter. Keyguard occlusion only applies to the default
display. To make the activity showWhenLocked flag work on secondary
displays, pass the display id of the locked secondary display from the
SystemUi to KeyguardController and make its isKeyguardShowing method
take a displayId.

Test: android.server.cts.ActivityManagerDisplayTests
Test: #testSecondaryDisplayShowWhenLocked
Bug: 64994006
Merged-In: Ib31fc76e9df469e97a59a181f09d457ceed4ef5f
Change-Id: Ib31fc76e9df469e97a59a181f09d457ceed4ef5f