SystemUI: Fix the unlock sound played repeatedly

Sometimes keyguardDone() may be invoked from PhoneWindowManager many times
in a short time, especially when app works with FLAG_DISMISS_KEYGUARD.

Make sure the unlock sound is only played when keyguard is showing and the
screen is on.

Change-Id: I396588579a3be3b3210e619179d2a57211904644
CRs-Fixed: 900840