Fix initial brightness state after startup.

Setting of mInitialAnimation member was assuming that currentValue
member would only be 0 at boot. It also gets set to 0 when the
sensor goes dark. This fix removes mInitialValue reliance on
currentValue.

Fixes bug 6444319.

Change-Id: I27069f1b44f1f727533d34bbecad2e5aeaa7aab8