WM: Prevent secondary display focus while keyguard is up

Fixes an issue where input intended for the keyguard could end up going
to a different display.

To prevent this, make sure that only the default display can get focused
when the keyguard is showing.

Change-Id: I6463c44aedca06930d2c9bda7c45ffd93141308c
Merged-In: I6463c44aedca06930d2c9bda7c45ffd93141308c
Fixes: 71786287
Test: atest DisplayContentTests