Fix flicker when device is unlocked from AOD.

When IActivityTaskManager.keyguardGoingAway is called, keyguard sleep
token should be removed to get relevant activities involved for the
keyguard going away app transition animation.

The old logic assumed that keyguard goes away only from the lock screen,
but it's also possible to do it from AOD screen.

Bug: 197113783
Test: atest DisplayContentTests
Change-Id: I5d6ab0170a6b2c92e0655bad85ca22ba322e7b31
(cherry picked from commit 43af33b2)