Simplify State initialization w/ better semantics + improved Task security.

Also, reveal the illusion that we're restoring state in Files and Downloads.
Also, define a "PairedTask" class that guards calls to task methods
    with checks against isDestroyed. This also let us make all of the tasks
    static, so we get much narrower scope and tasks can even be moved to
    their own files.

Change-Id: I6a9e8706e1ab1d1f43301e73dd9858a115a6baaf