SystemUI: Fix NPE of PreviewInflater during monkey test

PackageManager.resolveActivityAsUser() maybe return null, and
ResolveInfo.activityInfo also may be null.

If one of them is null, it would launch resolver activity. Add null check
refer to SecureCameraLaunchManager.targetWillWaitForCameraAvailable().

CRs-Fixed: 775522

Change-Id: Id94bdd376c0310588fcdb4b9a570ce4fff541bb2