Update the layer's alpha value upon composition of the layer

Fixes: This patch makes sure that the layer's alpha value is
up-to-date and does not reflect the previous view's alpha value.
It fixes the square block on marquee fading edge when text view
is applied transparency.
CRs-Fixed: 562000

http://review.cyanogenmod.org/#/c/54146

Change-Id: I97a9e7d98cd8db0c63f1907490d335e88075b1fa