Active display: Unify (un)registering callbacks and add proper checks.

Change-Id: Ie4c4660a54d2c60831f7a3bcc66b3a9ca1d035f2

Conflicts:
	packages/SystemUI/src/com/android/systemui/statusbar/policy/activedisplay/ActiveDisplayView.java

KANGER NOTE:

This also reverts commit 77e4b5462691f1baef23a0a59927129c12ef5dfd