AM: Clear keyguardGoingAway after it has gone away

Fixes an issue where the keyguard going away flag was not cleared
until the keyguard showed again, which prevented SystemUI from ever
controlling the orientation while it is not showing the keyguard,
such as during unlocked AOD.

Change-Id: I40d4052321fe76c988d673a5ecbc07b74192bec6
Fixes: 69368568
Test: Set up secure keyguard, Turn off "power button locks", open a landscape app, press power button, verify AOD shows in portrait.