Prevent momentary switch of keyguard to landscape

This fixes a bug where keyguard was created without setting
the preferred orientation.  The result is it would temporarily
assume the orientation of the device after being hidden.

The fix sets the flag when the keyguard view hierarchy is created
rather than when visibility changes.

Fixes bug 8124979

Change-Id: Ic54a185e4f51f79f2bb7fbc5ffc6992641e89132