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

Change-Id: I397308b739f5f4fe4b630cf594825f401c97e0e3

Conflicts:
	libs/hwui/OpenGLRenderer.cpp