Fail silently if a process trys to launch an acitvity for background user

Doing this so we don't break current apps. In the future we should
properly position the activity in the stack (i.e. behind all current
user activity/task) and not change the positioning of stacks.

Bug: 21801163
Bug: 13507605
Bug: 22929608
Change-Id: I979b6288e66f5b2ec2a6f22cb8d416e5c68109bd