Active display: Only fetch next notification if mNotification is null

Change-Id: Ife8511d4a4b8c448bc7aa8997a250068bec5fae7

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/policy/activedisplay/ActiveDisplayView.java
Active display: Don't show active display if call state is not idle.

This will prevent active display from displaying when a call is
coming in and let the call screen show up instead of active display.

Change-Id: I2cb79e097ae24dc6ad6c8a33be88eb19f7d8d235