Use nav bar color for invocation lights

Registers a listener so that the assistant invocation lights are
set to the same color as the nav bar (and therefore show up on
white backgrounds). Changes the stroke caps to BUTT rather than
ROUND/SQUARE so that they do not overlap (since the nav bar color
can be non-opaque).

If the lights are explicitly set to some other color, the listener
is not used.

Bug: 134523274
Test: manual
Change-Id: I4fc2dd4c609dcada6acdf34e269a0e62547747d0