Notify light bar controller when GONE

When the scrim state becomes GONE, send an alpha update
to the LightBarController that controls the 3 button nav
color. The ensures the color resets after the bouncer
disappears.

Fixes: 341190547
Test: atest ScrimControllerTest
Flag: EXEMPT bugfix
Change-Id: Iee76f2d6039a69b4bbc129d49be305694736ba8e