libstagefright: align YV12 for software codecs on exynos4

YV12 chroma stide should be aligned to 16 according to the
android framework. But, GSC & Mali couldn't accept a seperate
stride for U & V components. They just use Y_stride/2 as a
UV Stride.

Change-Id: Ibeeb3439ee10a4666420e5fa34ff0a6d6ea7c003