Ignore duplicate keyguardGoingAway request.

SysUI calls ActivityTaskManager#keyguardGoingAway to start keyguard
going away process. In case SysUI calls the binder call again while
keyguard is going away, ActivityTaskManager ignores it.

Bug: 215201490
Test: atest KeyguardTransitionTests
Change-Id: Iaee8c7cd25f0a89c5e90bd6fc0185dd54fb1d99e