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.

Change-Id: I711a81c3e79f7accdc1d9cb217b7806a0b8fcf63