Fix lockscreen button behaviour.

Unlike the other buttons, the lockscreen disable button keeps the state
(disabled/enabled) in it instead of just displaying it, so in order to
make it reliable, a few things needed to be fixed:

- The actual lockscreen blocker was a member variable while the state
  information was static. This could lead to the two variables getting
  out of sync.
- On orientation or theme change, all widgets were destroyed and
  recreated. If that happened while the lockscreen was disabled, the
  blocker instance was lost, thus, there was no way to enable the
  lockscreen again. Fix that by only recreating the views, not the whole
  widgets.
- The lockscreen disable state wasn't stored persistently, thus didn't
  survive a reboot.

Fixes http://code.google.com/p/cyanogenmod/issues/detail?id=2978.

Change-Id: I6f619740659cfedec3a3c9517fb55275f601d792