Fix system crash in Atms.notifyKeyguardFlagsChanged.

The crash happened when the external display removed during relayout and post
message to notify keyguard flags changed for updating transition on
this display, and then got NULL activiy display after message received.

Add a null check in notifyKeyguardFlagsChanged to fix this crash.

Bug: 119285175

Test: All existing tests passed.
Change-Id: I1768b1485d20973049fc92f97dcaccde31510499