libstagefright: back off exynos4_enhancements HWC_HWOVERLAY usage

The open source hwc doesn't support the
GRALLOC_USAGE_HW_FIMC1 | GRALLOC_USAGE_HWC_HWOVERLAY
combination. Removing HWC_HWOVERLAY allows FIMC1 to do
decode/color conversion.

Change-Id: Ibdfa9e5dd170e99e0d994540979e88e519931641