Prevent full wake lock from keeping device awake while dreaming.

A dream may itself hold a wake lock in order to keep the screen
bright as it runs.  However this wake lock also causes the device
to stay awake even when it is not plugged in which is undesirable.

This change makes full wake locks behave differently when napping
or dreaming.  The wake lock still keeps the screen bright but
it does not prevent the device from falling asleep.  This is
similar to our policy of ignoring full wake locks completely when
the device is manually put to sleep by the user.

Bug: 7295909
Change-Id: Id99e82d2143ae1a81629281d6407d7527efb8137