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

Change-Id: Ie4c4660a54d2c60831f7a3bcc66b3a9ca1d035f2
Active display: Catch Resources.NotFoundException

This caused the SystemUI to FC when the device was powered and a notification was fired.
Display stayed off and the SystemUI kept restarting until the device was unlocked manually.

Change-Id: Ibfd82242e82aa9ee5c0898f5f0714e490eac0ea9