Fix issue #5642121: Proximity sensor didn't turn the screen on in call

Turning animations back on exposed this.  The problem is that when the
screen brightness changes, it initiates a brightness animation.  When
we force the screen to black as we wait for it to be ready to display,
it sees that an animation is running so stops it and thinks this means
it should now turn the display off.

To fix this, don't modify the screen brightness while we are waiting
to show the screen.  This is good anyway because the whole point is to
avoid showing the screen until ready, and modifying the brightness at
that point would turn it on prematurely.

Change-Id: I84b296f8ca5705c2d237ea7741cdeb95c5521df9