Single-user restrictions

Introduces a new "blocked" state for each package. This is used to temporarily
disable an app via Settings->Restrictions.

PIN creation and challenge activities for use by Settings and other apps. PIN
is stored by the User Manager and it manages the interval for retry attempts
across reboots.

Change-Id: I4915329d1f72399bbcaf93a9ca9c0d2e69d098dd