Applications should -not- be able to set preferred activities.

I can't believe I let this slip through.  And in the SDK no less. :(

The APIs for setting preferred activities will now throw a security
exception when used.  This may break some apps, we'll see how it
goes.  If it is too bad we can just make these log and not throw
anything, but I would much prefer they throw an exception.

Change-Id: I3aed434750eef8b202aa9d5bd774a0121be521c6