Hide content on all displays when the device is locked.

Currently we cannot show Keyguard on private display. To prevent
security regression, we should hide the content when device is locked.

Bug: 120669490
Test: atest ActivityManagerMultiDisplayTests
Test: atest ActivityManagerDisplayLockedKeyguardTests
Test: atest ActivityManagerDisplayKeyguardTests

Change-Id: I94a5099b91f15b3871da497a8bc7cb66713d9b64