AOD: Fix shelf animating when it should not

Fixes an issue where the shelf would animate downwards when going to AOD.

The issue is that setDark gets called in updateIsKeyguard even though nothing changed,
yet it still re-requests the dark transition. Consequently, when the keyguard goes dark
later, the requested transition executes and moves the shelf downwards.

Change-Id: I8c590aeb4f330d8c3c1b51285d899d5f6db36323
Fixes: 64412244
Test: Double tap on notification on AOD2; press power; verify shelf does not move downwards