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
Allow creating custom dialogs in DialogPreference.

This allows creating non-AlertDialogs while still using the click and
state handling of DialogPreference.

Change-Id: I54fd7d6d6b9f6a49fef1ae95e0178838b3edfa56