Fix 4560303: Add setting to lock later when power button pressed

This adds a feature to delay locking the device when the power button
is pressed.  This fixes a use case where the user wants to turn off
the display (e.g. to save power) but doesn't want to lock the device.

For the case of a secure device (user has a pin/password/pattern),
this will lock the device immediately or not based on the setting.

For the non-secure case, this always "locks" the device to provide easy
access to the camera while preventing unwanted input.

Change-Id: Ie328485c3f7559e26896d761cbf0e69d3f4df4e2