Prevent unnecesary Surface preservation.

We don't save the Surface's pixel format, and so any time
there is a change in the pixel format as described in the params
we trigger a surface preservation. However if an app is switching
from forced translucent (due to HW accel) to explicitly translucent
even though the layout params have changed we don't actually need
to trigger a format change, just toggle the OPAQUE flag.

Bug: 38324871
Test: Launch Chrome Custom Tabs with Lateral Transition Animation from News and Weather App. No black bars. go/wm-smoke
Change-Id: I2151b4470fd7c395fba7aad7d6ffca4c51e55476