sf: Always use opengles for screen capture

Go back to the usage of GRALLOC_USAGE_HW_TEXTURE and GRALLOC_USAGE_HW_RENDERER
in captureScreenImplLocked regardless of useReadPixels value

This fixes the EGL_NO_IMAGE_KHR error returned from
eglCreateImageKHR (blank images returned from screenshot path)

Change-Id: I62fe90a081607b9e89c67f3dcfd34c84efc89d35