Support keeping activities resumed while dozing.

If the lock screen is not shown, then let activities keep running
while dozing.  This is important to support ambient mode on watches
since it allows the home app to keep running.

To make this possible, we need to inform the activity manager about
the exact wakefulness state, not just the overall boolean
interactive state.

Bug: 18284212
Change-Id: Ia35c99127ce51ffc178f3a2f51fca67d24061c71