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
Fix stencil buffer bug.

Stencil test is enabled by setStencilFromClip(), but not disabled
in some use case, which causes some of the pixels are discarded by
mistake in draws followed. Disable stencil test when there is no
dirty clip.

CRs-Fixed: 539456

Change-Id: I318fb47212e49e2da89bcbfd161a1689ef91b02f