hwui: Don't discard framebuffer on exynos4

The layers gets empty/not cleaned and redrawn on top of the preview,
causing beautiful glitch on homescreen, and drawing black instead
of transparency.

Change-Id: I633b104224c73c625f1193ee3dde41823cddc5bf
@hide API change from themes

Change-Id: Ia0798346d2e8e5a84e8456bb89b7d503a71a7afa

Conflicts:
	core/java/android/content/res/Resources.java