Add configurable nav bar opacity rules

NAV_BAR_OPAQUE_WHEN_FREEFORM_OR_DOCKED (default) - Nav bar is always opaque when
either the freeform stack or docked stack is visible.
NAV_BAR_TRANSLUCENT_WHEN_FREEFORM_OPAQUE_OTHERWISE - Nav bar is always
translucent when the freeform stack is visible, otherwise always opaque.

Bug: 26425669

Change-Id: I36ff12df4448a35c66f0e3dc2272169496b562d5