Added the concept of a "trusted" event to distinguish between events from
attached input devices or trusted injectors vs. other applications.
This change enables us to move certain policy decisions out of the
dispatcher and into the policy itself where they can be handled more
systematically.
Cherry pick of b931a1b4 from gingerbread into master.
Change-Id: I700a5f07b8b227878cea9437a289a45a245c0424
e20c9e0264