Don't scale screen brightness by electron beam level.

This change removes the modulation of the screen brightness
by the electron beam level.  The screen brightness remains
constant while the electron beam animation is playing.

Previously we were multiplying the screen brightness by the
electron beam level so as to animate both at the same time.
The problem is that when the screen brightness is already dim
to begin with, it may not be possible to see the electron beam
animation because the modulated screen brightness rapidly
converges on 0.  This may manifest give the appearance of
an abrupt transition or a flash as the screen turns off.

Bug: 7387800
Change-Id: I27b90f0098bbdc3de1d66fad819548d1301405cd