TrustInterfaceService: Init mNotificationManager in the constructor

Ensure that the variable is not null when removeNotificationForFeatureInternal(int)
is called so to avoid the NPE:

   E AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.NotificationManager.cancel(int)' on a null object reference

This fixes LineageParts crashing when disabling Trust notifications.

Change-Id: Ia0fa793378116101821a7bd424e0859869db48f5